Object Identification system with adaptive transceivers and methods of operation

ABSTRACT

An object identification system includes a monitor and a plurality of transceivers that communicate over a common medium. The monitor includes a first transmitter, a first receiver, and a processor. Each transceiver includes a resonant circuit, a transmitter, a receiver, and an antenna coupled to the resonant circuit. The processor performs a method for performing transceiver communication that includes the steps of: (a) transmitting from the first transmitter a first frequency for a first duration; (b) after lapse of the first duration, receiving via the first receiver a response signal from at least one of the resonant circuits; (c) determining a second frequency from the received response signal; and (d) performing transceiver communication using the second frequency. Transceivers of the type having a resonant circuit coupled to an antenna, when operating in close proximity to each other, may interfere with the response from a single transceiver by absorbing the energy intended to be received by the transceiver, absorbing the energy transmitted by the transceiver, or altering the resonant frequency of the resonant circuit. By determining the second frequency for transceiver communication, the monitor may establish communication with the single transceiver at a frequency better suited for transferring operative power to the transceiver, conducting an interrogation protocol for identifying the transceiver, or for data transfer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-In-Part application of, and claimspriority from, U.S. patent application Ser. No. 09/233,755 by Rodgers,et al., filed on Jan. 20, 1999, which is a Continuation-In-Partapplication of U.S. patent application Ser. No. 09/088,924, by Rodgers,et al., filed on Jun. 2, 1998, now abandoned. These related applicationsare incorporated herein by reference.

FIELD OF THE INVENTION

Embodiments of the present invention relate to communication systems ofthe type having multiple transmitting and receiving devices that share acommon communication medium; and, to methods for establishingcommunication in the presence of large numbers of such devices.

BACKGROUND OF THE INVENTION

Conventional data communication systems have been applied to accomplishobject identification using the medium of radio broadcast. Such radiofrequency identification (RFID) systems find application in the fieldsof materials handling, inventory control, and generally in the field oftracking personnel, objects, and animals. In an exemplary arrangement,such a system may include an interrogator and several thousandtransceivers, each transceiver being packaged as a disposable label ortag and placed on an object, animal, or person to be tracked. Eachtransceiver is manufactured using integrated circuit technology,programmed with a unique identifier, and assembled with a printedcircuit antenna to form a flat assembly for incorporation into the labelor tag. Typically, the interrogator has a fixed location, whiletransceivers are moved from time to time in and out of the communicationfield of the interrogator. It is highly desirable to accurately andquickly identify transceivers from a population of transceivers whichmay number in the billions. At the same time, it is highly desirable toreduce the cost of each transceiver to an absolute minimum.

Accurate and reliable detection of transceivers is made difficult by anumber of factors including, for example, (a) transceivers have alimited amount of power available to operate when required to respondwith a radio transmission; (b) the orientation of the transceiverantenna may be unsuitable for absorbing sufficient power from the signaltransmitted by the interrogator; (c) the orientation of the antenna ofthe transceiver may be unsuitable for providing a transmitted signalsufficient for accurate reception by the interrogator; (d) cooperationof a transceiver with the interrogator may require sophisticated logicin the transceiver to accurately perform the transceiver's portion of acommunication protocol used to obtain an open communication channelbetween the interrogator and a single transceiver; and (e) transceiverstransmitting simultaneously may cause a so-called collision.

There remains a need for a communication system suited for coordinatingthe use of a common medium among potentially billions of transceiversfor interrogation or control activities to be accomplished in a limitedtime. In addition, there remains a need in some applications to minimizethe circuitry, firmware, and software complexity required at eachtransceiver, to extend the operating range of communication, and tosupport larger numbers of individual identification numbers perhaps atthe expense of complexity at the interrogator. Without theseimprovements, the size and cost per transceiver cannot be reduced topermit new and improved communication systems that employ inexpensivedisposable transceivers such as identification tags, baggage tags,inventory labels, and the like.

SUMMARY OF THE INVENTION

A system in one implementation according to various aspects of thepresent invention includes a monitor and a plurality of transceiversthat communicate over a common medium. The monitor includes a firsttransmitter, a first receiver, and a processor. Each transceiverincludes a resonant circuit, a transmitter, a receiver, and an antennacoupled to the resonant circuit. The processor performs a method forperforming transceiver communication that includes the steps of: (a)transmitting from the first transmitter a first frequency for a firstduration; (b) after lapse of the first duration, receiving via the firstreceiver a response signal from at least one of the resonant circuits;(c) determining a second frequency from the received response signal;and (d) performing transceiver communication using the second frequency.

Transceivers of the type having a resonant circuit coupled to anantenna, when operating in close proximity to each other, may interferewith the response from a single transceiver by absorbing the energyintended to be received by the transceiver, absorbing the energytransmitted by the transceiver, or altering the resonant frequency ofthe resonant circuit. By determining the second frequency fortransceiver communication, the monitor may establish communication withthe single transceiver at a frequency better suited for transferringoperative power to the transceiver, for conducting an interrogationprotocol for identifying the transceiver, or for data transfer.Communication is maintained in spite of variation in the resonantfrequency of the resonant circuit which may arise from coupling asdiscussed above or from variation in manufacturing and operatingenvironment (e.g., temperature, humidity, relative movement, orcomponent aging).

The monitor may further include a first antenna coupled to the firsttransmitter and a squelch circuit for dissipating energy on the antennaafter lapse of the first duration and before receiving from the firstreceiver the response signal from the resonant circuit. By quicklydissipating energy, the response signal may be more quickly andaccurately received by the second receiver and consequently the secondfrequency may be more quickly and accurately determined, increasingsystem sensitivity and reliability. Obtaining quicker receiving from thesecond receiver extends the operating range of the monitor or permitsoperation with weaker signals. Weaker signals may originate fromtransceivers located further from the monitor or in an orientation thatis detrimental to reception by the first receiver. Such detrimentalorientation of the antenna in the transceiver may be with respect to thefirst antenna of the monitor or with respect to other transceiversproximate to the transceiver antenna.

The monitor may further include the second receiver providing phasedetection, or a signal analyzer provides phase detection. Phasedetection providing phase information regarding the received responsesignal. The processor may further determine the second frequency inaccordance with the phase information. Phase information varies over awider range of values near a resonant frequency. By determining thesecond frequency in accordance with phase information, the secondfrequency may be more accurately determined. Communication with a moreaccurate second frequency improves the efficiency of transferringoperative power to a transceiver, permits faster or more accurateidentification of transceivers, extends the operating range of themonitor, overcomes problems of detrimental orientation discussed above,or permits faster or more accurate data transfer between the monitor anda single transceiver.

When each transceiver has a respective identification number comprisinga common total number of portions, a method of determining anidentification number of a transceiver of a plurality of suchtransceivers in one embodiment according to various aspects of thepresent invention includes the steps of: (a) transmitting a startsignal; (b) receiving a reply at a time after the start signal; (c)determining a number in accordance with the time determined in step (b);(d) transmitting a start signal and the number determined in step (c);(d) repeating steps (b) through (d) until a count of performances of thestep of transmitting is not less than the common total; and (f)determining the identification number in accordance with each reply.

By repeating the steps of transmitting a number of times not less thanthe common total, a step of detecting whether a collision occurred isnot necessary. The reply may convey no more information than the factthat a reply has been made, thereby eliminating the need for a longerduration of reply. By dividing an identification number into portionsand applying the protocol discussed above, a large number of uniqueidentification numbers is practical (e.g., 2⁴⁰ in 4 10-bit portions)without increased complexity or cost in each transceiver.

A short reply duration is associated with several advantages. Morereplies may be received in a given time period, increasing thelikelihood of identifying transceivers that are only briefly in range ofthe monitor; redundant replies may be used to increase systemreliability; and the amount of power needed in each transceiver totransmit a reply may be reduced.

Lower power consumption is associated with several advantages,including: transceivers with lighter weight, smaller size may bepractical at lower cost; and the communication range may be extended byexpanding the power budget used for receiving or transmitting or both.

Extending the communication range has additional advantages, including:increasing the time permitted for communication for transceivers thatare only briefly in range; decreasing the adverse affects of detrimentalorientation as discussed above; permitting closer proximity betweentransceivers; permitting larger numbers of transceivers in closeproximity to each other; reducing the size of antennas; and decreasingthe number of monitors or antennas that may otherwise be needed toprovide communication in a large area.

The method of determining an identification number may include a stepfollowing step (b) for rejecting an invalid reply. Further, time domainor frequency domain techniques which may be employed in the process ofdetermining a second frequency in the method for performing transceivercommunication may be used in the process of determining anidentification number in the step of rejecting an invalid reply.

A transceiver in one implementation according to various aspects of thepresent invention includes a resonant circuit (having a resonantfrequency), a receiver, a memory, a comparator, a counter, and atransmitter. The resonant circuit includes an antenna used for receivingand transmitting. The receiver, coupled to the resonant circuit detectsa start signal followed by indicia of a first code. The comparatorprovides a result of comparison responsive to the first code and asecond code provided by the memory. The counter is loaded with a countprovided by the memory and provides a completion signal after a durationin accordance with the count. The transmitter transmits a reply inresponse to the result of comparison and the completion signal.

When the second code maps to a transceiver identification number, such atransceiver identification number may be determined without thetransceiver transmitting the second code. The duration of transmittingthe reply is, therefore, brief with advantages as discussed above.

When such a transceiver is used with the system described above and theresonant circuit is used to establish the frequency for transmitting,the first receiver of the monitor may selectively receive in a reducedfrequency band expected to include the reply. Improved receiversensitivity with concomitant improved range of reception results.

A transceiver may further include a phase locked loop that locks to thefrequency being received, maintains the locked frequency in the absenceof received signal, and drives the transmitter to transmit at themaintained frequency instead of the resonant frequency. Improved rangeof transmitting by the transceiver may be obtained. Improvedcommunication may be obtained as a consequence of being able to provideoperative power, determine identification, and provide data transfer ata frequency different from the resonant frequency particularly when theresonant frequency is being affected by detrimental orientation asdiscussed above.

By transmitting a reply in response to the completion signal, a numericvalue may be communicated from the transceiver to the monitor with anumeric resolution in accordance with the duration from the startsignal. For example, multi-bit digital values may be communicated with a1-bit reply.

A monitor in one implementation according to various aspects of thepresent invention includes a processor for communication with aplurality of transceivers, an event detector, a plurality of receivers,a plurality of transmitters, and an antenna network controller forcoupling the monitor to a provided antenna network. The processor mayinclude a first and a second processor coupled for data transfer by acomputer network. The processor may determine the location of atransceiver in a zone monitored by an event detector in response to asignal provided by the event detector in cooperation with transceivercommunication as discussed above. Multiple receivers providesimultaneous narrow band detection for receiving a signal in accordancewith a predetermined phase. Multiple transmitters provide each ofmultiple simultaneous or sequential transmissions, each on a respectiveantenna (or group of antennas) and at a respective amplitude, frequency,and phase which may vary from other respective transmissions.

An antenna network in one implementation according to various aspects ofthe present invention includes a plurality of antenna nodes coupled toan antenna bus. Each antenna node includes a plurality of transceiverchannels and a coupler for coupling each transceiver channel to aprovided plurality of antennas. Each transceiver channel includes asquelch circuit. When the squelch circuit is located proximate to apoint in each of several antennas, out of band energy related tosquelching is reduced. In another implementation, the squelch circuitincludes a plurality of current sources for each of leg of an antenna tobe squelched.

An antenna network node in another implementation according to variousaspects of the present invention includes a cross-channel coupler and atransceiver channel that includes a difference amplifier for signalprocessing proximate to provided antennas.

An antenna network in another implementation according to variousaspects of the present invention includes an antenna bus, and aplurality of network nodes each comprising a processor, a tuner, and acoupler for coupling provided antennas to the tuner. The bus conveys asignal having indicia of a command with settings. The processor directsoperation of the tuner in accordance with the settings. In anotherimplementation, a conductor of the bus conveys at a first time indiciaof the command and at a second time indicia of a signal to betransmitted.

A passage in one implementation according to various aspects of thepresent invention includes planar antennas each arranged at a respectiveangle to provide in combination a minimum received signal greater than apredetermined amount for all possible orientations of a transceiver inthe passage. In an alternate implementation, each antenna includes a Qmodifying circuit that facilitates wider-band reception thantransmission.

A carrier in one implementation according to various aspects of thepresent invention includes an antenna and a series capacitor for tuningthe antenna. Enhanced transceiver communication results whentransceivers are placed in the carrier. In an alternate implementation,a carrier includes a first and a second antenna each with a respectivetuning capacitor. The first and the second antenna are coupled tocooperate. Energy received in a first pattern is re-radiated in secondpattern for further enhanced transceiver communication.

BRIEF DESCRIPTION OF THE DRAWING

Embodiments of the present invention will now be further described withreference to the drawing, wherein like designations denote likeelements, and:

FIG. 1 is a functional block diagram of an object identification systemin an exemplary embodiment according to various aspects of the presentinvention;

FIG. 2 is a functional block diagram of an exemplary implementation ofthe transceiver portions of objects 104 and 105 in the system of FIG. 1;

FIG. 3 is a graph of signal property magnitude verses frequency, for thepopulation of objects 102 through 112 in the system of FIG. 1;

FIG. 4 is a timing diagram of signals 170 and 172 in a transmission andresponse scenario of the system of FIG. 1;

FIG. 5 is a flow diagram of a method for data communication between amonitor and one or more transceivers of the system of FIG. 1;

FIG. 6 is a flow diagram of a method for performing the scan step of themethod of FIG. 5;

FIG. 7 is a flow diagram of a method for performing the subscan step ofthe method of FIG. 5;

FIG. 8 is a data flow diagram of processes performed by each transceiverin an exemplary implementation of the system of FIG. 1;

FIG. 9 is a chart describing the purpose and scope of various commandsgiven by a monitor and performed by a transceiver in the system of FIG.1;

FIG. 10 is a chart describing the structure and effect of a set ofcommands in an implementation of the system of FIG. 1;

FIG. 11 is a message format diagram describing message formats used toestablish and carry out data communication in an exemplaryimplementation of the system of FIG. 1;

FIG. 12 is a flow diagram of a method for performing the step ofinterrogation in the method of FIG. 5;

FIG. 13 is a flow diagram of a method for performing the “send commandand stack replies” step of the method of FIG. 12;

FIG. 14 is a flow diagram of a method for performing the “list members”step of the method of FIG. 12;

FIG. 15 is a timing diagram of signals related to interrogation in anexemplary implementation of data communication for the system of FIG. 1;

FIG. 16 is a timing diagram of signals for demodulating a receivedsignal and for modulating a signal for transmitting in a transceiver inthe system of FIG. 1;

FIG. 17 is a functional block diagram of a rectifier of a transceiver asin FIG. 2;

FIG. 18 is a functional block diagram of a receiver of a transceiver asin FIG. 2;

FIG. 19 is a functional block diagram of an alternate detector for thereceiver of FIG. 18;

FIG. 20 is a functional block diagram of a transmitter of a transceiveras in FIG. 2;

FIG. 21 is a functional block diagram of an alternate transmitter for atransceiver as in FIG. 2;

FIG. 22 is a functional block diagram of a state machine of atransceiver as in FIG. 2;

FIG. 23 is a functional block diagram of a memory of the state machineof FIG. 22;

FIG. 24 is a functional block diagram of a monitor of the system of FIG.1;

FIG. 25 is a functional block diagram of a receiver of the monitor ofFIG. 24;

FIG. 26 is a functional block diagram of a diode detector of thereceiver of FIG. 25;

FIG. 27 is a functional block diagram of a synchronous detector of thereceiver of FIG. 25;

FIG. 28 is a functional block diagram of a transmitter of the monitor ofFIG. 24;

FIG. 29 is a functional block diagram of an antenna node of the systemof FIG. 1;

FIG. 30 is a functional block diagram of an RF channel of the antennanode of FIG. 29;

FIG. 31 is a functional block diagram of a tuner of the antenna node ofFIG. 29;

FIG. 32 is a functional block diagram of a squelch circuit of theantenna node of FIG. 29;

FIG. 33 is a functional block diagram of an antenna network interface ofthe antenna node of FIG. 29;

FIG. 34 is a chart describing various planar antennas with reference tothe geometry of the passage of FIG. 35;

FIG. 35 is a plan view of a passage through which objects of FIG. 1 maypass for purposes of identification and control in an exemplaryinstallation of the system of FIG. 1;

FIG. 36 is a schematic diagram of an antenna of the system of FIG. 1;and

FIG. 37 is a plan view of a carrier which may be used to enhancecommunication for several objects of the system of FIG. 1.

In each functional block diagram, a broad arrow symbolically representsa group of signals that together signify a binary code. For example, theoutput of a binary counter is represented by a broad arrow because abinary count is signified by the signals on several conductors takentogether at an instant in time. A group of signals having no binarycoded relationship may be shown as a single line with an arrow. A singleline between functional blocks conveys one or more signals. Signals thatappear on several figures and have the same mnemonic are coupledtogether by direct connection or by additional devices.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An object identification system, according to various aspects of thepresent invention, provides communication between a monitor and anobject, while the monitor and object are within communicating range.Each object includes a resonant circuit coupled to an antenna used forcommunication. Communication, as used herein, may be used to accomplishone or more purposes including: (a) to detect presence of a resonantcircuit (e.g., to locate an object as in a zone), (b) to provideoperative power to a transceiver, (c) to determine the resonantfrequency of such a resonant circuit, (d) to determine a transceiveridentification, (e) to receive data from a transceiver, or (f) to senddata to one or more transceivers. Transmitted power levels may varyaccording to the range suitable for the communication. For example,objects may be detected at a higher transmitted power level and awarning issued that some objects may be out of range for interrogation.Communication may be accomplished using the same or different media orfrequencies for different purposes (e.g., magnetic induction, radio,infrared light, or acoustics). Different media or frequencies may beused simultaneously or at different times for the same purpose. Whensuch objects are proximate to each other, the antennas couple theresonant circuits to provide a corporate resonant frequency, typicallylower than the resonant frequency of each resonant circuit in isolation.According to various aspects of the present invention, communication isestablished, overcoming the problems described above including variationin the orientation of each object antenna and coupling effects (e.g.,proximity of object antennas to each other, and surfaces that interferewith communication by reflection, absorption, or refraction). Forexample, object identification system 100 includes host computer 122,network 128, monitors 124 and 126, antenna systems 120 and 122, sensors160 and 162, and controls 164 and 166. System 100 is capable ofestablishing reliable communication in spite of interference fromtransmitting sources not part of system 100. For example, interferencesource 190 (representative of any number of sources and locations)broadcasts signal 193 (representing one or more frequency components, ornoise) within the reception range of antenna systems 120 and 121.

Host computer 122 may include any computer system having computingcapacity and interfaces for supporting data communication on network 128among one or more monitors 124, 126. A conventional office computersystem may be used. Host computer 122 may operate to receive notice ofobjects detected or identified by monitors 124, 126 and to conduct anyotherwise conventional business process in response to such notice. As arepresentative example, host computer may provide inventory accounting,point of sale services, materials handling, automatic data collection,electronic article surveillance, or electronic access control inresponse to object detection or identification where objects may includepersonnel badges, identification tags, transportation tags, inventorylabels, electronic keys, authorization devices, or price tags.

Network 128 may include any network for data transfer (e.g., aninternet, a wide area network, a local area network using cable,telephony, or wireless technology) between a monitor and a hostcomputer. In addition, network 128 may support data transfer between oneor more monitors 124, 126.

Host computer 122 may perform a significant proportion of the dataanalysis, communication (e.g., formation and analysis of messages to andfrom objects according to one or more protocols for determiningidentification), and control functions discussed herein with respect toa monitor, when, for example, a monitor 124, 126 is of limitedprocessing capability. In such an implementation, monitor 124 receivescommands from host computer 122 and provides reports to host computer122 via network 128. Commands may include requests by host computer 122for the current state of controls 164, the current readings from sensors160, the status of any antenna node 140, 142, and the status of theconfiguration of monitor 124 or antenna system 120. Host computer 122may command monitor 124 to transmit on one or more desired frequencies,may direct monitor 124 to receive on one or more bands (wide or narrow)and/or perform analog and digital analysis of signals received fromantenna system 120, and may direct reconfiguration of monitor 124,sensors 160, controls 164, and/or antenna system 120. Further, hostcomputer 122 may, by suitable commands, request notice of objectsdetected or a list of object identifications currently withincommunication range of monitor 124, and/or request raw data from whichhost computer 122 may detect objects or determine such a list. Finally,host computer 122, using suitable commands to monitors 124 and 126, maydirect cooperation of monitors 124 and 126 for performing any of thefunctions discussed above.

A monitor includes any system that communicates with one or more objectsand provides results of such communication. Results may be provided toan operator at the monitor (e.g., when host computer 122 is omitted) orto a host computer for processing as discussed above. System 100 mayinclude one or more monitors, several monitors being used for redundancyor when the capacity of a single monitor is exceeded by physicaldistribution of objects or the desired extent of communication with anexpected population of objects in perhaps a limited time. For example,monitors 124 and 126 may be functionally equivalent and arranged in twogeographic zones or territories. When redundant communication withobjects by each monitor is not desired, the location of an object aswithin a particular zone may be ascertained by communication with one ofthe two monitors 124 or 126. Movement of an object from one zone toanother may be determined by host computer 122 from suitable reports bymonitors 124 and 126.

An antenna system includes any system for coupling one or more antennasto a monitor for communication between a monitor and one or moreobjects. When communication from one or more of several monitors islimited to providing operative power, receiving antenna functions ofthose monitors and antenna systems may be omitted. For example, forcommunication as discussed above, antenna system 120 includes antennabus 132 coupling antenna node 140 and antenna node 142 to monitor 124.Antenna node 140 supports antennas 150. Antenna node 142 supportsantennas 152. In like manner, antenna system 121 includes antenna bus136 for coupling antenna node 144 and antenna node 146 to monitor 126.Antenna node 144 supports antennas 154. Antenna node 146 supportsantennas 156. As used herein, an antenna represents any transducer ofenergy used in communication including, for example, a lens for infraredlight energy or a horn or structure for acoustic energy. An alternateantenna system includes one or more replaceable modules forreconfiguring operation from any communication medium or frequency bandto another medium or frequency band.

An antenna bus includes any network for conveying signals for couplingone or more transmitters to one or more antennas, for conveying signalsfor coupling one or more antennas to one or more receivers, and forcoupling one or more processors for data communication. For example,antenna bus 132 couples antennas 150, 152 to transmitters and receiversof monitor 124. In addition, antenna bus 132 couples processors inantenna nodes 140 and 142 with a processor of monitor 124. Monitor 124may direct antenna node functions and receive status information byissuing commands to one or more antenna nodes via antenna bus 132. In analternate implementation, more than one monitor may use the same antennabus. For example, monitors 124 and 126 may be coupled for communicationvia antenna bus 132 in place of (or in addition to) communicationbetween monitors via bus 128.

Communication between a monitor and an object may involve one or moreantennas. For example, communication between monitor 124 and object 103is illustrated with signals 170 from antennas 152 to object 103; and,signal 172 from object 103 to antennas 152. It is not necessary for thesame antenna node to operate for sending and receiving communication toa particular object. For example, antennas 152 provide signal 174 toobject 102; and, object 102 provides signal 176 for reception byantennas 150.

The orientation of an object antenna, as discussed above, includes theorientation of the object antenna with respect to an antenna used by amonitor for communication with objects and includes the orientation ofthe object antenna with respect to other object antennas. Whenessentially planar antennas are used in the monitor and objects,coupling of antennas for power transfer from a monitor to an object maybe primarily by magnetic fields. Such coupling may decrease as theobject antenna orientation differs from coplanar (or parallel planes)with respect to the monitor antenna. When planar object antennas arecoplanar (or in parallel planes) with respect to each other, an objectmay receive power from other objects and the coupling of multipleresonant circuits may effect the behavior of one or more of suchresonant circuits. For example, when each object has a resonant circuitwith a resonant frequency when operated in isolation, a group of objectsmay have a peak of energy absorption at a different (e.g., lower)frequency, herein called a stack resonant frequency. Some objects in astack may not be coupled to the same extent as other (e.g., a majority)objects and so may absorb energy more efficiently at a frequency betweenthe resonant frequency in isolation and the stack resonant frequency ofthe majority. In other words, a nonuniform stack of objects may exhibitseveral stack resonant frequencies.

The cooperation of resonant circuits in such a system of coupled objectantennas may have a detrimental effect on communication. Detrimentaleffects may include insufficient operative power being received by aparticular transceiver in an object so that other purposes ofcommunication cannot be met; insufficient or discontinuous power tosupport digital and analog functions (e.g., counting, sensing,converting) so that data communication may be inaccurate; limited rangeof a signal transmitted by an individual object; and a different thanexpected power spectral density of a signal transmitted by an individualobject.

Sensors 160, 162 measure various aspects of the environment near therespective monitor, while controls 164, 166 effect changes in thatenvironment. Sensors 160, 162 may include any conventional electronictransducers including, for example, temperature sensors, pressuresensors, proximity sensors, electromagnetic sensors, optical sensors,and mechanical sensors such as used conventionally for detectingenvironmental physical conditions, movement of objects in a surveillancearea, opening and closing of doors, and passage of vehicles, animals,personnel, and/or items not equipped with transceivers. In animplementation of system 100 for automatic data collection related to apoint of sale terminal, sensors 160, 162 may include a bar code reader,a video camera, and other conventional product tracking sensors.Controls 164, 166 may include any conventional facility controls whenmonitors 124, 126 are stationary; or, may include vehicular controls, asappropriate, for monitors 124, 126 in a mobile configuration. Controls164, 166 may include controls for changing the orientation of one ormore antennas of antenna systems 120, 121. Each monitor 124, 126integrates and reports information related to events as detected bysensors 160, 162 and related to communication with one or more objects102 through 112. Such reports may be provided by alarms, speechenunciators, printouts, or displays (not shown). Each monitor 124, 126may respond to one or more detected events by changing the state ofcontrols 164, 166 and/or reporting one or more events across network 128to host computer 122 and/or another monitor.

Sensors and controls as discussed above may be supported in an alternateimplementation of system 100 from one or more antenna nodes in additionor in place of sensors 160 and 164 supported from monitor 124 directly.When supported by an antenna node, sensors and controls may be placed inlocations distant from monitor 124 or more suitable for signal routing,system installation, test, or maintenance. A node of such animplementation may support any combination of antennas, sensors, andcontrols, including configurations of exclusively antennas (as shown),sensors, or controls.

System 100 may be constructed and assembled using conventionalelectrical and electronic components and techniques including firmwareand software developed using conventional software developmenttechniques. Objects for use with system 100 may be constructed andassembled using conventional electrical, electronic, and mechanicaltechniques including packaging as integrated circuits, hybrids, smartcards, labels, tags, badges, packing materials, packaging, receptacles,or signage as desired for any of the applications discussed above.Although the physical proximity of objects is illustrated in FIG. 1 forclarity, the functional block diagram of FIG. 1 is not intended toconvey other physical aspects of system 100. Any of various physicalpackages and distributions of the functions of system 100 may beemployed using conventional packaging and data communication technologyfor desired system operation. For example, the functions of hostcomputer, monitor, and antenna system may be integrated in one packageor partitioned into numerous cooperating or redundant packages. System100 may be expanded to include any number of host computers (one shownfor simplicity), any number of monitors (two shown for simplicity), andany number of antenna nodes per antenna system (two shown forsimplicity). Antenna system 120 may be integral to a single location,distributed within one or more zones, or mobile. Similarly, objects102-112 may have relatively fixed locations (e.g., embedded in roadways,moving belts, etc.) when monitors are mobile or portable.

Objects 104 and 105 form stack 114, wherein respective object antennasare coupled to some extent (e.g., more or less aligned in parallelplanes or coplanar and/or positioned in more or less close proximity toeach other). Likewise, objects 107 through 112 form stack 116. Forobjects having planar antennas operating at from 1 to 15 MHz, couplingsufficient to observe a stack resonant frequency different from theresonant frequency of an isolated object may occur at distances betweenparallel aligned object antennas less than 8 inches (e.g., about 1inch). Stack 114 of objects 104 and 105 cooperate as described belowwith reference to FIG. 2. Each object 104, 105 includes an identicaltransceiver 201, 231. Transceiver 201 includes antenna 202, tank circuit204, rectifier 206, receiver 208, transmitter 210, and state machine212.

Tank circuit 204 is a conventional resonant circuit (e.g., a series,parallel, or series/parallel resonant circuit). The inductance ofantenna 202 may cooperate with tank circuit 204 as an additionalinductance or as the primary inductance of tank circuit 204. Antenna202, when located proximate to antenna 232, may be joined by lines offlux indicated generally as 290. Lines of flux 290 represent magneticcoupling between antennas 202 and 232. The effects of magnetic couplingon tank circuit 204 include (a) change to the resonant frequency of tankcircuit 204, (b) change to the Q of tank circuit 204, (c) loading oftransmitter 210 when transmitting, and (d) attenuation of any signal(e.g., power or message) received by receiver 208. When tank circuit 204receives energy for the purpose of providing power to transceiver 201,magnetic coupling may decrease the energy received for conversion topower by rectifier 206. Tank circuits 204 and 234 cooperate when coupled(e.g., ring currents in phase, one resonant frequency herein called thestack resonant frequency, and energy sharing). Particular advantages areobtained in system 100 as a consequence of enhancing some of theseeffects and accounting for these effects in the functions performed bymonitor 124 and/or host computer 122. For a transceiver operative at 8to 10 MHz preferably at about 5.5 MHz) tank 204 may have a Q in therange 90-130 in isolation, 40-70 when coupled to transceiver circuitry,and as low as 20 when proximate to other transceivers. For example, astack of from 3 to 100 transceivers in coplanar orientation may have a Qof about 35.

Each monitor 124, 126 may at any suitable time perform a method forselecting one or more frequencies (or bands of frequencies) forcommunication between the monitor and one or more objects. Uponselecting a frequency (or band) for communication, monitor 124 mayproceed further to detect, empower, interrogate or transfer data withone or more transceivers by transmitting and/or receiving messages usingthe selected frequency (or band). For example, method 500 of FIG. 5 maybe performed by monitor 124, or by the cooperation of host computer 122and monitor 124 as discussed above. A monitor may provide power totransceivers at any time with respect to other communication (e.g.,prior to interrogation, interleaved during interrogation, simultaneouslyon another frequency, or not at all for battery powered transceivers).

At step 502, a sequence of frequencies in a desired scan range isdetermined and stored in an array of monitor transmit frequencies forscanning, MTFS [1 . . . A]. Such a sequence of frequencies may includeany integer number of frequencies (e.g., as indicated by the variable A)and may be selected from (or stored in) array MTFS in any suitableorder. Preferrably, a sequence of frequencies is selected so as to avoidtransmitting more than a predetermined average power in any particularband of frequencies. A frequency range may be divided into any number ofbands. Such bands may be of any bandwidth, may overlap, and may omit oneor more portions of the range. The sequence of frequencies may providefor one or more transmissions in a first band followed by one or moretransmissions in any other band. For example, transmission on afrequency in a first band (e.g., F308 in band F304 to F312) may befollowed by transmission of any frequency in a second band (e.g., F324in band F320 to F328) to limit average power transmitted in the firstband. A frequency offset from the beginning of a band may be used as anoffset in another band; although, differing respective offsets in eachband may be used. For example, any order of frequency transmissiondescribed in related patent application Ser. No. 09/088,924, cited abovemay be used.

Scanning may be defined for a range about a center frequency dividedinto an integer number of contiguous bands of identical bandwidth. Forthe purpose of limiting average power transmitted in each band, scanningmay be accomplished in a number of subscans. Each subscan may includeone transmission in each band at an offset from the lower boundary ofthe band. The subscan may proceed from band to band in sequential orderof increasing frequency. The offset used in a first subscan may beincreased by an incremental amount for use in a subsequent subscan. Thenumber of subscans performed may depend on whether a frequency ofinterest or candidate frequency is detected (as discussed below); or thenumber of subscans may be equal to the number of transmissions to bemade in each band. Given all of the above constraints, the frequencyused in each transmission may be expressed by the formulae:f(s, t) = [n(s, t) × (2R/N)] + (F − R)${n\left( {s,t} \right)} = {\left( {s + {\left( {N/T} \right)(t)}} \right)\overset{{({N/T})} - 1}{|\limits_{s = 0}}\overset{({T - 1})}{\underset{t = 0}{|}}}$

F is the midpoint frequency (e.g., in MHz);

F±R is the range of frequency to be scanned;

2R/N is the increment in frequency (e.g., in MHz);

N is the total number of transmissions in the range to be scanned;

T is the total number of transmission in a subscan;

n is the frequency number for each transmission;

s is the subscan number within each scan; and

t is the transmission number within each subscan.

In the scanning technique described by the above formulae, N, T, n, s,and t may all be integers to facilitate computation (e.g., loop countersand limits). Values for s and t may be consecutively selected asintegers from the series of integers indicated by the bounds in theabove formulae.

In alternate scanning techniques, any series may be used in place of theseries of integers, for example, a series of real numbers may be used.Any function may be used to determine a next value of the series,including, for example, a pseudo random number generator. When bands arenot treated consecutively, are not of equal bandwidth, or are notcontiguous, any algorithm (e.g., a look up table, or set of rules) maybe used to determine suitable values for a next frequency to be used fortransmission. Similarly, a suitable offset to be used in each subscanfor each band may be determined by any suitable algorithm. For example,a pseaudo random number generator may be used to determine a next bandand a next offset for a next transmission in that band. The amplituteand/or duration of each transmission may vary, for example, as afunction of frequency, when average power is to be limited into areactive or resonant load (e.g., a load that is not purely resistive). Anext frequency that is determined according to a series or algorithm asdiscussed above, may be omitted from a subscan as a consequence offorecasting the average power that would be transmitted in the band anddetermining whether a maximum average power would be exceeded if thetransmission were not omitted. Such a determination may include anaccounting for prior transmissions over a suitable time period.

When different operating frequencies are used for differentcommunication purposes as discussed above (e.g., an object may have aresonant circuit for receiving power and a second resonant circuit forinterrogation), frequencies for scanning may be chosen in any sequencefor determining any combination of operating frequencies of one or moreobjects. For transceivers operative in isolation at about 5.5 MHz,scanning may include frequencies in a range from about 2.5 MHz (e.g.,F304) to about 6.0 MHz (e.g., F328) to account for manufacturingtolerances and object orientation (e.g., stacks) as discussed above.

Array MTFS may include, for each frequency, values that specify theconfiguration to be used for transmitting and receiving. Such values mayspecify configuration parameters for each transmitter (e.g., powerlevel, synchronization, duration, one or more antennas, tuning, anddriving phases) and for each receiver (e.g., selection of detector,selection of clocking signals, filter parameters, synchronization, oneor more antennas, tuning, squelch timing, and signal processingparameters as discussed below). For efficiency, default values orreferences to sets of predefined values may be used. Filter parametersand/or signal processing parameters may effect selective attenuation ofinterference (in time domain or frequency domain) as determined in anyprior execution of a step of method 500. Because both transmit band andreceive band may be specified for each entry in array MTFS, alternatescanning techniques may be used including: (a) transmit a narrow bandsignal and receive with a wide band detector; (b) transmit a wide bandsignal and receive with a narrow band detector; (c) transmit two or morenarrow band signals (consecutively or simultaneously) and receive with awide band detector; or (d) maintain transmitting of a wide band signalwhile receiving at consecutive times with different narrow band detectorsettings.

At step 504, a scan subroutine is performed in accordance with thecontents of array MTFS. Any suitable method of scanning may be used fordetermining one or more signal properties of candidate frequencies tofacilitate selecting one or more frequencies for interrogation.Particular advantages are obtained in system 100 by use of a scan methodof FIG. 6. Control may be transferred from step 504 to step 601 of FIG.6.

At step 602, the first monitor transmit frequency for scanning isselected from array MTFS using a loop variable S that is assigned thefirst index value 1.

At step 604, unmodulated carrier at the frequency indicated by the valueMTFS[S] is transmitted from antenna system 120 (e.g., one or moredefault antennas, or one or more antennas determined in step 502discussed above) for duration D430 illustrated as signal 170 in FIG. 4.Carrier transmission begins at time T410 and continues until time T414.The rise and fall time of the unmodulated carrier may be substantial asshown in FIG. 4 or (preferably) may be negligible. The duration D430 ispreferably short in comparison to a START signal discussed below. Fulloperation of transceivers 201, 231 is not required during scanning. In apreferred scanning method, carrier transmission is insufficient toprovide operative power in any transceiver.

At step 606, one or more antennas (e.g., those used in antenna system120 for the transmission of carrier in step 604) may be squelched forduration D434 to stop radiation which may interfere with receiving onthe same or different antennas. The antenna squelch function iseffective on or near a zero crossing of signal 170, as shown at timeT414, to avoid transmitting out-of-band noise. The squelch operation iscomplete at time T416. The duration D434 is preferably less than oneperiod of the frequency being transmitted at step 604 (e.g., from aboutthree periods of the transmitted carrier to less than 1 microsecond,preferably from 1 to 3 μsec). Antennas not in use are squelched or leftopen to avoid detection of an antenna resonant frequency at step 608.

Energy transmitted by signal 170 (e.g., a magnetic field), when receivedby one or more transceivers 201, 231, will consequently develop anoscillating (i.e., ringing) current in tank circuits 204, 234 andantennas 202, 232. Each oscillating current will persist after time T414as a consequence of the Q of the tank circuit. For example, as anoscillating current passes through antenna 202, a ring signal istransmitted from antenna 202 from time T416 to time T422. Signal 172 ofFIG. 4 illustrates in an approximate fashion the extent of the ringsignal. When lines of flux 290 couple one or more tank circuits, allcoupled tank circuits cooperate. Consequently, signal 172 may includethe superposition of signals from one or more separate objects and/orone or more stacks, as described above. Signal 172 is typically severalorders or magnitude lower in amplitude than signal 170. Signal 172 mayalso differ in frequency and phase from carrier signal 170. Thesedifferences in frequency and phase, as well as changes in amplitude ofsignal 172 between times T416 and T422 convey information about tankcircuit 204, about the orientation of transceiver antenna 202 withrespect to antenna system 120 and other transceivers, the number ofsimultaneously ringing tank circuits, and possibly the location andrelative movement (e.g. within a zone) of tank circuits with respect toantenna system 120.

At step 608, signal 172 is received by antenna system 120 (e.g., one ormore default antennas, or one or more antennas determined in step 502discussed above) and sampled for duration D436 between times T416 andT418. Although a shorter duration may be used, the duration T416 to T418and the sensitivity of the receiver (at signal levels expected to bereceived in a particular application) are selected to provide about 8periods of the ring signal for sampling. A number of samples 417 arerecorded in monitor received scan array MRS[1 . . . D]. Each sample mayindicate an amplitude of signal 172 (e.g., a measured analog voltageconverted to a digital representation). In addition, samples may betaken at time T418 through time T422 for further analysis.

At step 610, various signal properties are determined in accordance withthe contents of array MRS and similar arrays corresponding to priorperformances of step 610. Any conventional signal property may bedetermined. A particular signal property may be determined at a firsttime (A) and again at a second time (B) (during the expected decay timeof a ring signal) and the relationship between signal propertymagnitudes at A and B may be used to determine a third signal property.The analysis of signal properties may proceed in the time domain (e.g.,amplitude, phase) or in the frequency domain. Analysis in the frequencydomain may proceed from the result of a conventional fast Fouriertransform (FFT) of a series of samples (e.g. a sampling window of 5 to50 μsec) taken beginning at time A (e.g., time T416 for 5 μsec) and/orbeginning at time B (e.g., time T418 for 5 μsec). Examples of suitablesignal properties are described in Table 1. In an alternateimplementation samples are taken at another time C after time B. Valuesof samples at times A and C are then normalized by dividing (e.g.,A′=A/B and C′=C/B) or by subtracting (e.g., A′=A−B and C′=B−C). Times A,B, and C may be arranged at even time intervals within the expectedduration of a response signal or reply signal.

TABLE 1 Signal Property Description A−B A signal amplitude at time A isexpected to be greater than a signal amplitude taken at time B. If not,the signal being analyzed may be interference, for example signal 193.The amount of the difference in amplitude should fall within anacceptable range. The range is based on the Q of tank circuit 204 and/orother coupled tank circuits, and the effect of simultaneous ring signalsfrom several coupled or uncoupled transceivers. A/B The ratio of anamplitude taken at time A to an amplitude taken at time B provides analternate indication of the Q of the ringing tank or tanks, as discussedabove. The ratio is expected to fall within a range of Q values for tankcircuits and stacks to be encountered by system 100. The A/B techniquemay provide more reliable results than the A−B technique at low noiseconditions. A (at freq¹) vs. A (at freq²) The signal amplitude at eachof two or more frequencies (when normalized) provides information thatis expected to be consistent with the Q of the ringing tank, or tanks,as discussed above. The difference between the normalized amplitude atany frequency when compared to an expected amplitude (based on a rangeof Q), if not within or acceptable range may indicate that one or moresignal amplitudes correspond to noise or interference. Phase at time BThe phase of the signal at time B is expected to correspond to the phaseof a decaying sinusoid of phase known at time A. The phase may bedetermined in any manner including, for example, comparing signals frommultiple receivers each having a phase sensitive detector, locking aphase-locked loop at time A for use at time B, or using digital signalanalysis. When the phase at time B differs from the predicted phase bymore than a predetermined amount, the signal may be interference. FFT(A)vs. FFT(B) Frequency components of the result of an FFT analysis mayindicate one or more significant component frequencies. The magnitude offrequency components of an FFT taken at time A should not differ morethan a predetermined amount from the magnitude of correspondingfrequency components of an FFT taken at time B. FFT(A at freq¹) vs.FFT(A at An FFT resulting from transmission at a first frequency isfreq²) expected to have frequency components that correspond tofrequency components of an FFT resulting from transmission at a seconddifferent frequency. When the respective amplitudes of correspondingcomponents do not differ more than a predetermined amount, thenon-different component may be a component of an interference signal,for example, signal 193. Phase(A at freq¹) vs. Phase(A A tuned circuitring signal is expected to exhibit a strong phase to at freq²) frequencyvariation at frequencies near the resonant frequency. If the phase doesnot vary by more than a predetermined amount as measured at a first anda second frequency, the signal may be interference. A_(a1−a2) = A(usingantenna 1) − An amplitude signal (e.g., at time A or B above) may bemodified A(using antenna 2) by subtracting the signal as received frommore than one antenna. Common mode rejection results. The modifiedsignal technique may be used in place of any non-modified signals in anyof the properties discussed above (e.g., A_(a1−a2) − B_(a1−a2);A_(a1−a2)/B_(a1−a2); FFT(A_(a1−a2)); etc.) A_(d1−d2) = A(using wide-bandBecause a ring signal is a narrow band signal, a wide band detectordetector 1) vs. A(using narrow- and a narrow band detector are expectedto provide signals of band detector 2) similar amplitude in a low-noiseenvironment. If the environment is known to be low-noise and the wideband detector produces a signal amplitude that exceeds by more than apredetermined amount the signal amplitude produced by a narrow banddetector, the signal being received by both detectors may beinterference.

At step 612, each signal property determined in step 610 is stored in anarray at an index position corresponding to the transmitted frequency atstep 604. For example, several arrays for monitor reply signalproperties may be indexed using the loop variable S, as MRSP1[S],MRSP2[S], etc.

At step 614, the loop variable S is incremented and a subsequent monitortransmit frequency is selected until all monitor transmit frequencieshave been transmitted. When a next monitor transmit frequency has beenselected successfully, control passes to step 604; otherwise controlpasses to step 616 for a return to the calling routine, for example,following step 504.

At step 506, arrays MRSP1, MRSP2, etc. are analyzed individually and/orby comparison and/or correlation to determine which frequency orfrequencies correspond to maximum values of a figure of merit based onone or more signal properties. Correlation may be time coherent orspectral coherent. For example, if a figure of merit is based solely ona single signal property as illustrated in FIG. 3, a conventional arrayanalysis may be used to determine that frequency F324 corresponds to amaximum signal property S384. Here, the graph of values shown in FIG. 3may be represented in memory as a list (or array) of frequency-propertypairs including, for example, (F304,S360), (F308,S380), and numerouspairs in between. Peaks values of the signal property may be noted inthe analysis, including frequencies F308, F312, F316, and F320. Furtheranalysis may determine one or more candidate frequencies in accordancewith conventional profile recognition logic and profiles of expectedsignal properties based on theoretical models, measurements, andanalysis. For example, if frequency F324 corresponds to the tankfrequency expected for a transceiver operating individually, thenfrequency F324 would be a candidate. By profile recognition frequencyF320 may be determined to correspond to stack 114 and frequencies F308,F312, and F316 may correspond to stack 116. Using the signal propertyvalue S384 for normalization, it may be determined that signal propertyvalue S380 corresponding to frequency F308 is also a candidate becauseits relative amplitude meets or exceeds a threshold value. However,signal amplitude S378 and S374 corresponding respectively to frequencyF312 and F316 may be of little interest based on the possibility thatthese minor peaks in signal property value may correspond to object 107and 112 (or similarly situated objects) having weaker coupling to allother objects 108 through 111 of stack 116 due to being positioned atthe respectively ends of stack 116. In other words, frequencies F308,F312, and F316 may correspond to a single stack 116 which may beinterrogated at a single frequency, for example frequency F308.Communication may be conducted at frequencies F312 and F316intentionally for one or more purposes (e.g., transmitting operativepower), for example, when it is expected that each frequencyrespectively corresponds to a different one or more transceivers (e.g.,a transceiver detuned for any reason including proximity to anothertransceiver or to a surface that interfaces with communication asdiscussed above).

In addition to the analysis of maximum values of the signal propertyshown in FIG. 3, further analysis may account for the Q (e.g., qualityfactor or standard deviation) of the signal property at each peakfrequency. For example, signal property at frequency F324 exhibits ahigh Q; signal property at frequency F308 exhibits a somewhat lower Q;and, signal property at frequency F320 exhibits a relatively low Q. Somefrequencies initially considered candidates may be eliminated when thevalue of the signal property (or figure of merit) does not correspond toa Q greater than a minimum expected Q, or the relative magnitude of thesignal property value does not exceed a minimum expected magnitude. Inthe case of stack 116 which may exhibit a signal property havingmultiple peak values as illustrated at frequency F308, F312, and F316,further analysis may be employed to determine which of the threepossible candidate frequencies is most suitable for interrogation.

In a controlled environment, signal properties may indicate the numberof objects present, improper orientation of one or more objects, orimproper spacing between objects.

A step 508, one or more candidate frequencies may be subject to furtheranalysis in conjunction with a subscan procedure. For each candidatefrequency a suitable range of frequencies proximate to the candidatefrequency is specified for a subscan. Any subscan procedure may be used.Particular advantages are obtained in system 100 by performing thesubscan procedure in a manner similar to the scan procedure discussedabove with reference to step 504. For example, control may transfer fromstep 508 to step 701 of FIG. 7. Frequency values specified for a subscanin array MTFS may be accompanied by any of the configuration values(e.g., revised for this subscan) discussed above with reference to Step502.

At step 702, a sequence of frequencies within each desired subscan rangeis determined. Values in an array of monitor transmit frequencies forscanning are determined, for example MTFS[1 . . . C]. A typical subscanmay span a frequency range of ±200 KHz around a frequency of interest.

At step 704, a scan procedure is performed in accordance with thecontents of array MTFS. Control may transfer to step 601 and returnsfrom step 616 as described above.

At step 706, revised signal property arrays MRSP1, MRSP2, etc. are usedto revise one or more figures of merit as discussed above with referenceto step 506. Control returns at step 708 to the calling routine, forexample, step 510 of FIG. 5.

At step 510, each frequency associated with a figure of merit having anappropriate magnitude is identified in an array of monitor transmitfrequencies for interrogation, for example, MTFI[1 . . . B].

At step 512, each interrogation frequency is used in the conduct of aninterrogation scenario. Any interrogation protocol and modulation methodmay be used. Suitable interrogation protocols are described in TABLE 2.An interrogation protocol for use in system 100 includes anyconventional protocol for the transfer of an individual identificationfrom a transceiver to a monitor, as well as any protocol from which amonitor may determine an individual identification. Subsequentcommunication employing the individual identification may then proceedwithout collision, interference, or ambiguity in system operation. Anymessage format and modulation method may be used, preferably anarrow-band modulation, for example, any pulse width modulation (PWM)technique.

Transceiver identification may include the frequency (or frequency band)in which communication can be reliably established, a code or sequenceof codes recognized by the transceiver for enabling one or more replies,a code indicated in (or by) a reply, or a combination of these features.

TABLE 2 Protocol Description 1 Time for beginning transmission of replymessage may be determined by the object's transceiver according to arandom number to decrease probability of collision. Long reply messagesmay be used. Object or monitor (or both) may include a mechanism forcollision detection to initiate retry. Content of reply message mayconvey identification. 2 All objects may have an assigned reply slotnumber. Object identification may be communicated in N parts in thecorresponding reply slot in reply to N requests. Content of reply slotmay convey identification. 3 Objects may reply redundantly in more thanone reply slot in reply to a single request. Collision detection may beused by the monitor to determine whether data in a particular reply slotis valid. Content of one clear reply slot may convey identification. 4 Aparticular address or a group address may be sent with the interrogationmessage. Objects may reply when addressed in particular or as members ofthe requested group. Failure to be addressed may impose an initial state(e.g., reset), preventing further replies. Context of a particularaddress may be implied from immediately preceding group address(es).When addressed, a reply in a particular reply slot may indicate a nextaddress in a predetermined sequence. Being addressed may silence repliesafter a predetermined minimum number of replies (e.g., one). Content ofreply slot may serve for reliable detection or for additionalidentification. 5 A relatively long identification number may be brokenup into several shorter access codes, each access code associated with alevel. Objects may be addressed in any sequence of access codes. Whensufficient access codes have been received, a reply in a particularreply slot may indicate an access code for another level in apredetermined sequence, a final portion of the identification number, ordata provided to the monitor. Groups of objects may be programmed withidentical access codes at predetermined levels for obtaining replies ina particular reply slot indicating an access code at a predeterminedlevel. Content of reply slot may be for reliable detection, additionalidentification, or providing data to the monitor. 6 Presence ofindividual and coupled groups of object transceivers may be determined.Identification may be determined in part by a frequency of a responsesignal. A group of transceivers may be enabled (turned on) in accordancewith one frequency of response. Each reply time slot may be assigned ordirected to be self-assigned (e.g., randomly). Reception of replies maybe restricted to a narrow band (e.g., notch) to ignore objects not inthe desired group. Time slots may be read in one or more messagesaddressed to subgroups. Identification may be determined from slotnumber of reply and/or content of reply. A subgroup may be turned off orautomatically disabled. Interrogation may continue for another subgroupor frequency until all portions of identification have been determined.7 Any combination of techniques illustrated by the above protocols maybe used in full or in part.

Particular advantages are obtained in system 100 using the interrogationprocedure 512 described in FIGS. 12, 13, and 14. Control may transferfrom step 512 to step 1201 with reference to FIG. 12.

The selected frequencies at step 510 may be used for interrogation, or,alternately, these frequencies may be used for transferring power frommonitor 124 to one or more objects 102 through 112. In this latter case,interrogation may proceed in any conventional manner on any suitablefrequency. For example, an object of the present invention having a tankcircuit that cooperates with the tank circuit of proximate transceiversmay receive energy from a broadcast at a frequency that is near theresonant frequency of the tank circuit. Further, such a transceiver mayrespond and participate in an interrogation scenario at anotherfrequency (e.g., 250 MHz to 350 MHz) using conventional RFID. Theinterrogation protocol and transmission modulation techniques used inconventional RFID include, for example, frequencies selected forsuitable propagation characteristics, infrared and other opticalfrequencies, and ultrasonic and other audio frequencies. Magneticcoupling between proximate transceivers as described above withreference to FIG. 2 may be obtained at any frequency suitable for thedimensions of antennas and distances between antennas for the desiredcommunication purpose. Magnetic coupling is preferred for providing apower signal so as to limit the range of the power signal to meetregulatory guidelines.

Modulation techniques include, for example, spread spectrum, amplitudemodulation, frequency modulation, single side band modulation, andoff/on keying (OOK) modulation. OOK is preferred for its narrowfrequency spectrum, permitting communication in the presence andorientation of other objects that absorb portions of wider-bandmodulation to an unpredictable extent.

According to various aspects of the present invention, the complexity ofcircuits and firmware for performing the functions of a transceiver maybe reduced by employing one or more of the following techniques incombination: (a) receiving operative power for the transceiver via theantenna and tank as described above; (b) employing transceiver detection(e.g., detecting a ring signal) at the same frequency that is used topower the transceiver; (c) employing OOK modulation for interrogation;(d) conducting interrogation at the same frequency as used for poweringthe transceiver; (e) limiting the reply from a transceiver duringinterrogation (e.g., one or two bits); (f) employing multiplepredetermined reply slots for multiple transceivers to reply to a singlecommand; (g) using predetermined durations of unmodulated carrier forone or more transceiver reset operations; (h) employing a transceiveridentification number of sufficient resolution to practically reduce thepossibility of collision in an expected operating environment to anegligible amount (e.g., possibly to zero); (i) employing a protocolthat identifies when a reply corresponds to exactly one transceiverwithout relying upon collision detection mechanisms; and (j) employing atransceiver identification number divided into N parts and employing aprotocol for ascertaining a part of an identity in more than onedifferent sequence of interrogation messages.

The functions of monitor 124 and transceiver 201 will be described belowin an implementation that includes all of the techniques listed above.Although any implementation of hardware, firmware (e.g., state machinemicrocode), or software (e.g., microprocessor instruction code) may beused to perform that portion of the protocol assigned to thetransceiver, an exemplary implementation supports interrogation andfurther supports read/write data communication. For example, the process800 of FIG. 8 may be performed by a transceiver 201, in order to supportsuch a protocol. Process 800 includes processes for the detection ofSTART and SEPARATOR signals 802; awaiting an access code 804; changingan access state 810; comparing an access code to an access code frommemory 806; transmitting a reply in a reply slot in accordance with anaccess state 812; awaiting a command 814; and transmitting a message816.

These processes may be supported in any combination of software,firmware, or logic circuits. Execution of these processes may proceed inan interrupt driven, polled, single threaded, or multitasking parallelexecution manner. As discussed below, a process notifies another processin any conventional manner, for example using a common variable, givinga command, producing a signal, etc.

Process 802 continuously analyzes received carrier for indications of aSTART signal and a SEPARATOR signal. Uninterrupted, unmodulated carrierfor more than a first predetermined duration may indicate a STARTsignal. When a START signal is received, process 804 may be notified.When a START signal is detected, the state of the transceiver should bereset to a known initial condition. Process 802 provides such notice toprocess 810 to reset the access state. Uninterrupted, unmodulatedcarrier for a second predetermined duration (preferably less than thefirst predetermined duration), may be used to indicate a SEPARATORsignal. A SEPARATOR signal, as used herein, may indicate an interruptionin a message and thereby indicate the onset of a subsequent message.Upon detection of a SEPARATOR signal, process 802 provides notice toprocess 812 to terminate any transmission which may be in progress orscheduled to be transmitted. Process 802 to process 812 provides noticeto reset the slot count accordingly. Failure to receive a proper STARTsignal may leave transceiver 201 in a power-off, reset, condition.Failure to receive a proper SEPARATOR signal may leave the transceiverexpecting the completion of the current message format.

Process 804 examines incoming demodulated carrier beginning from anotice of a START signal until a predetermined time when an access codeis expected according to the message format. A protocol supported byprocess 800 divides the individual transceiver identification into oneor more access codes. Each access code is associated with a so-calledlevel code to be discussed below. Process 804 passes the received levelcode and access code to process 806 on receipt.

Process 806 operates on a valid received level code and access code whenprovided by process 804. Process 806 uses the level code as an addressor index into an array stored in memory 808 to retrieve a stored accesscode. Further, process 806 compares the stored access code with thereceived access code and provides results of that comparison to process810 in various protocols supported by process 800. Note that it may notbe necessary for access codes to be received in any particular sequencein as much as each access code is received with an associated level codefacilitating an appropriate access code to be retrieved from memory inaccordance with the level code. Alternatively, any suitable sequence maydictate a desired access state change, as discussed below.

Process 810 changes the access state of transceiver 201. In response toa reset state command (or signal) from process 802, process 810 resetsall access state bits. When a suitable result of comparison is receivedfrom process 806, process 810 may set one or more access state bits.Preferably, process 810 sets an access state bit in accordance with thelevel provided by process 804 when a suitable result of comparisonindicates that the received access code exactly matched the storedaccess code. Various alternate protocols may permit or require an accessstate bit to be set when a result of comparison indicates anyconventional relationship between the received access code and thestored access code (e.g., >, >=, <, <=, within a range, etc.). Process810 provides the current access state to process 812 and, upon obtaininga predetermined access state, may command process 814 to begin acommand/reply session.

Process 812 is enabled to transmit when the access state provided byprocess 810 meets or exceeds a predetermined enabling access state(i.e., the transceiver has been addressed to any extent defined by theprotocol). Process 812 retrieves a slot count from memory 808 inaccordance with the level code provided by process 804. According to apreferred protocol supported by process 800, slots (designated withpredetermined counts) follow the occurrence of a START signal by apredetermined delay. Transmit process 812, after lapse of thepredetermined delay, counts predetermined slot time durations (or slotboundary signals) until the slot count is achieved. Process 812 thentransmits a reply signal in the slot corresponding to the slot countretrieved from memory. By transmitting a reply signal in a predeterminedreply slot, process 812 as executed in multiple identical transceivers,provides a reply that, on receipt by monitor 124, indicates that one ormore transceivers have been enabled to transmit as a consequence ofhaving received one or more suitable access codes.

Each access code may represent a group (or subgroup) identificationnumber. When arranged hierarchically, the individual transceiveridentification may consist of a (GID) number, a subgroup identificationnumber (SGID), a sub-subgroup identification number (S²GID), etc. to anynumber of levels. For example, when each access code represents a 10-bitbinary number, and four levels are used, an individual transceiveridentification number consists of a 40-bit binary number. Thisidentification number is sufficient to identify uniquely more than onebillion transceivers in each of more than 1,000 independent operatingenvironments. Each operating environment is identified by a 10-bit groupidentification number (e.g., a top level access code) with 30 bitsremaining for identification of individual transceivers.

Process 814, upon notice of a begin session command, from process 810,performs any suitable command/reply protocol which may differ instructure and function from the interrogation protocol described abovewith reference to processes 802 through 812. The command/reply protocolmay include commands to send data to a transceiver and to obtain replydata from a transceiver beyond the 1 bit transmit capability discussedabove with reference to process 812. Process 814 may store received datain memory 808 and provide a command to process 816. Process 814 maycontinue for multiple command/reply exchanges until: (a) operative poweris no longer provided (or commanded to be removed) by monitor 124; (b) acommand addresses and changes one or more enabling access state bits inone or more transceivers; or (c) the completion of a command by atransceiver is accompanied by an automatic change of one or moreenabling access state bits.

Process 816 receives a command from process 814 and may recall datastored in memory 808 and/or obtain measurement data from a conventionalsensor (not shown). Data from memory and/or one or more sensors may betransmitted by process 816 in any suitable manner in accordance with theprotocol discussed above with reference to interrogation, the protocoldiscussed above with respect to a command/reply session, or anyconventional protocol.

In an implementation of system 100 wherein data transfer to and from atransceiver is not required beyond the capability to identify thetransceiver, processes 814 and 816 may be omitted and suitablesimplifications made to process 810. On the other hand, a protocolsupported by process 800 may include a variety of commands as discussedbelow with reference to FIG. 9. For purposes of interrogation andidentification of an individual transceiver identification, commands 904and 912 may represent a minimum configuration.

Commands 902, 904, and 906 affect the access state of a transceiver.Command 902 resets an access state bit. Command 902 may be omitted in asimplified variation, where resetting all access state bits isaccomplished by ceasing to supply operative power to a transceiver.Because power is supplied from monitor 124 by broadcasting carrier, theneed remains in some implementations of system 100 to reset one or moreparticular access state bits in a group of transceivers or in onetransceiver without affecting the access state of unaddressedtransceivers. Command 902 in combination with one or more access codeswill provide the facility for resetting one or more access state bits asdefined in a conventional manner by suitable additional codesaccompanying (or integral with) the command.

Command 904 is used to set an access state bit in one or a group oftransceivers. As discussed above, command 904 may be used to accumulatea sufficient number of prerequisite set access state bits in order toenable process 812. In a variation of the interrogation protocoldiscussed above, command 904 may be used to set any arbitrary pattern ofaccess state bits, perhaps in a predetermined sequence, to facilitateany purpose of communication as discussed herein.

Command 906 is used to clear the slot counter in all transceivers. Byclearing the slot counter, this command assures that no further replieswill be attempted by transceivers without the occurrence of a subsequentcommand, possibly including further access codes sufficient to obtainthe access state required for operation of process 812. Command 906 maybe omitted in a system implementation wherein no message is terminatedbefore such message is allowed to proceed to completion. In a systemusing command 906, efficiencies may be obtained by clearing the slotcounters when all expected (or significant) replies have been received.

Commands 908 and 910 accomplish sending data to transceivers frommonitor 124. Command 908 may be used to transfer data from monitor 124for storage in memory 808 in one or an addressed group of transceivers.Command 908 may require a prerequisite access state for groupidentification, security, or reliability purposes. Command 910 may beused to configure one or more sensor configuration registers so as tocontrol any conventional aspect of sensor operation (e.g., the time ameasurement is begun, the duration during which a measurement is taken,the resolution or accuracy of the measurement, designation of anymeasurement analysis, etc.).

Commands 912 through 920 may be used to obtain data from a transceiver.Command 912 may be used in the interrogation protocol as discussed aboveto indicate the existence of an addressed transceiver. In response tocommand 912, a transceiver may reply with a 1-bit acknowledgement in areply slot corresponding to that transceiver's respective membership.For example, if a group of transceivers is addressed, each transceivermay reply with an acknowledgement in a respective reply slotcorresponding to that transceiver's membership in a particular subgroupof that group. When fully addressed (i.e., no subgroup is defined belowthe lowest level of the current state of the interrogation scenario),the transceiver receiving command 912 may reply with an acknowledgementin a respective reply slot corresponding to its identification number(e.g., the least significant portion of the identification number, i.e.,a member identification number). As discussed above, command 912 may becombined with command 904 to the effect that when a reply is made tocommand 912 an access state bit is also set. Particular advantages areobtained in system 100 by providing command 912 in a form with thesetting of an access state bit (as in command 904) and in another formwherein no access state bit is affected.

Commands 914 and 916 may require that the command be directed to atransceiver that has been fully addressed so as to assure that only onetransceiver will attempt to respond to the command. For example,assuming data from memory and sensor data exceeds one bit in length, onetransceiver can reply with data from its memory in response to command914 (or one transceiver can reply with sensor data in response tocommand 916) without collision, only when monitor 124 has identified onetransceiver to send the data and has fully addressed only thattransceiver. The length of data to be supplied in one or more replies tocommands 914 and 916 may vary. Without departing from the generalstructure of a series of reply slots as discussed in the interrogationprotocol discussed above, up to 1,000 bits of memory or sensor datacould be provided from a transceiver in reply to a single command 914 or916. Such data may be provided in redundant or differential redundantformat to assure reliable reception by monitor 124.

Commands 918 and 920 demand a reply from one or a group of transceivers.The reply to command 918 may consist of one dibit, two redundant bits,or a short sequence of bits (e.g., preferably one bit) in each replyslot corresponding to data from memory. In a system having 1,000 replyslots, 1,000 transceivers may respond with one bit each until all bitsof data from memory have been provided. In like manner, the reply tocommand 920 may provide data from up to 1,000 sensors with one bit persensor in each reply slot. In an alternative protocol, commands 918 and920 are directed to a fully addressed transceiver. Such a transceiverprovides a reply from which a 10-bit memory value or sensor value may bedetermined. By replying in a reply slot corresponding to the appropriatevalue (e.g., 1 to 1,000), a 1-bit reply specifies a decimal number toone part in 1,000. When 1024 reply slots are used, a 1-bit reply conveysa 10-bit binary value. A command/reply session may be used to accomplishone or more of the functions described in Table 3.

TABLE 3 Purpose Command/Reply Session Tracking An identification of themonitor may be written into transceiver memory by a suitable command andmay include monitor location (if not implicit), monitor operatoridentification (if any), and time-date or process codes (e.g., materialshandling or manufacturing step). Replies may indicate time-date whenlast addressed, monitor identification when last addressed, or listedhistory of time-date and monitor identifications when addressed. Repliesmay be restricted in scope to one or more commands of interest (historyof changes to sensor configuration, changes to identification, etc.).Security One or more access codes (at one or more levels) may be revisedwith a suitable command sequence including confirmation of the new code(e.g., repeat what was commanded or send partial or completeidentification) prior to enabling use of the new code, and directing useof the new code. Alternately, a programmed set of alternate access codesmay be enabled. These techniques may be used to implement code hopping.Any of the identification features discussed above may be confirmed,rewritten, or subject to selection among predetermined alternatives byone or more suitable commands to accomplish re- identification of one ormore transceivers. For example, transceiver VCO center frequencies maybe reassigned and/or tank properties may be modified (e.g. byintroduction of switched elements, digital control, or other tuningtechniques).

One or more of the purposes described in connection with commands 902through 920 above may be accomplished by particular message formats in aset of messages optimized for use in a particular instillation of system100. For example, command formats 1004 through 1007 of FIG. 10 may besufficient to provide interrogation and identification of up to 1billion transceivers in 1,000 applications as discussed above.Particular advantages obtained in system 100 according to variousaspects of the present invention by expanding the set of commands toinclude commands 1000 through 1003 of FIG. 10. The expanded set ofcommands may be used during interrogation, assembly, or test todetermine, for example, a 40-bit transceiver identification numberwithout proceeding through a hierarchical interrogation sequence. Forexample, each command 1000 through 1003 provides an argument identifyinga group identification number. No prerequisite access state bits must beset. No access state bits are set as a consequence of receiving thecommand. And, the reply from each transceiver is similar to the replydescribed with reference to command 912, except that transceivers willrespond with a sub-group identification number to command 1000; willrespond with a sub-sub-group identification number to command 1001; willrespond with a sub-sub-sub-group identification number in reply tocommand 1002; and will respond with a sub-sub-sub-sub-groupidentification number in reply to command 1003. Commands 1000 through1003 may be used to (a) determine or confirm the complete identificationof a physically isolated transceiver; (b) determine or confirm all orpart of an identification number of one transceiver when all othertransceivers have been disabled; (c) quickly estimate the number oftransceivers within communication range; (d) quickly detect thepossibility that transceivers may have moved into or out ofcommunication range; or (e) confirm that a particular subgroup oftransceivers is not within communication range.

In contrast to commands 1000 through 1003 which do not set an accessstate bit, commands 1004 through 1007 each set an appropriate accessstate bit. In addition, commands 1005 through 1007 may reset thetransceiver access state logic if the prerequisite state bit is notalready set.

In an exemplary interrogation scenario, command 1004 is first providedwith a level 1 group identification number in order to obtaininformation as to level 2 sub-group memberships of all addressedtransceivers. The reply slots indicate the level 2 sub-groupidentification number of those transceivers addressed by the groupidentification number. In addition, state bit B0 of access state logicis set. The level 1 group identification number is preferably a 10-bitaccess code. The level 2 sub-group identification identified by a replyslot indicates a 10-bit access code. Second, command 1005 provides thelevel 2 sub-group identification number as its argument, and elicits thelevel 3 sub-sub-group identification number from addressed transceiversthat are members of the group identification and sub-groupidentification as indicated by prerequisite state bit B0 and successfulcomparison of the provided sub-group identification number and the level2 access code retrieved from memory. As a result of successfullycompleting command 1005, transceivers that are members of the group andsub-group will set state bit B1 corresponding to level 2. Third, command1006 is provided with level 3 sub-sub-group identification number as anargument. Transceivers having successfully passed commands 1004 and 1005will have set the prerequisite state bits B0 and B1. A reply to command1006 provides the level 4 sub-sub-sub-group identification numberindicated by the corresponding numbered reply slot. Further, accessstate bit B2 is set corresponding to level 3. Fourth, command 1007provides the level 4 sub-sub-sub-group identification number as theargument and elicits in the respective reply slot the memberidentification number of those transceivers that have successfullypassed comparison of the group identification number, sub-groupidentification number, and sub-sub-group identification number asindicated by prerequisite state bits B0, B1, and B2 being set, and,further, successful comparison of the sub-sub-sub-group identificationnumber provided with command 1007 and the level 4 access code retrievedfrom memory. That transceiver that has successfully replied to command1007 will also set access state bit B3. A system manager of a system 100may arrange transceiver identification numbers so as to assure that theidentification number provided by commands 1004 through 1007 will alwaysaddress exactly one transceiver. In a variation of system 100 supportingcommands 908, 910 and 914 through 920, command 1008 may be used with anappropriate argument to read or write data into memory or aconfiguration registration of a sensor or read data from a sensor orfrom memory as discussed above. A reply to command 1008 (e.g., in aparticular reply slot) may provide a write acknowledgment or provide a10-bit data value from a sensor or memory location as discussed abovewith reference to command 918 and 920. Further setting of access statebits may be unnecessary for command 1008. In a variation, further accessstate bits may be defined and set by various commands of the typedescribed above with reference to command 1008 to accomplish moresophisticated transceiver functions.

The commands and arguments discussed with reference to FIG. 10 may bearranged in message formats in any conventional manner. Particularadvantages are obtained in system 100 according to various aspects ofthe present invention, using the message formats of FIG. 11. Forexample, message format 1100 consisting of a binary code identifying acommand 1101 may be used to accomplish commands 902 and 904. Thesecommands require no argument when the command identifying codeimplicitly identifies one or more access state bits.

Message format 1110 may be used for commands 914 and 916. Message format1110 includes command identifying code 1111, pad 1113, and reply bits1114. Pad 1113, when used, conveniently separates command code 1111 fromreply bits 1114 and assures reliable recognition in the transceiver ofthe first reply bit of reply bits 1114. Reply bits 1114 may include anynumber of bits in serial format.

Message format 1120 may be used for commands 912, 918, and 920 discussedabove. Message format 1120 includes command code 1121, pad 1123, andreply slots 1125. Reply slots 1125 identify numbered periods of time.Each slot being used for a reply. A reply may consist of one or morebits, however, 1-bit reply slots are preferred. In an alternate replyslot configuration, a 1-bit reply may be presented as a dibit consistingof the reply bit in both true and compliment form.

Message format 1130 includes command code 1131, argument 1132, pad 1133,and reply slots 1135. Argument 1132 may be any binary code. For example,argument 1132 may convey a level code and an access code as discussedabove.

Message format 1140 may include command code 1141, argument 1142, pad1143, and separator 1146. Separator 1146 may include uninterrupted,unmodulated carrier as discussed above. In contrast, pad 1143 mayinclude a period of time during which no carrier is transmitted.

In the message formats described above, command codes 1101, 1111, 1121,1131, and 1141, are of identical structure. Likewise, pads 1113, 1123,1133, and 1143, are of identical structure and may provide delay forprocessing a received command and argument. Reply slots 1125, and 1135are of identical structure and function. Arguments 1132, and 1142 maybeof identical structure or may vary as desired and indicated bycorresponding command codes.

An example of a method to conduct an interrogation at monitor transmitfrequencies of interrogation according to step 512 is presented belowwithin the context of process 800 executing in each transceiver. Controlmay transfer from step 512 to step 1201 of FIG. 12 for performance ofthe interrogation method of FIGS. 12 through 14.

At step 1202, three variables are set to initial conditions. Variable Cis set to 0 to indicate a command of the form 1000 of FIG. 10 is to beissued. Variable RS is set to 1 to indicate a first reply stack is to beused to store replies. Variable G is set to a group identificationnumber of interest. Variable G may be a suitable structure for numerousvalues as discussed below. When a group identification number is used todistinguish one of 1000 installations of system 100, the groupidentification number may correspond to a customer number, a geographicarea, a political territory, and/or any arbitrary indication thatuniquely specifies this installation for the purpose of eliminatingconfusion with transceiver identification numbers that are properlymembers of a different system installation. Commands 1000 through 1007are identified by values 0 through 7 of variable C, respectively.

At step 1204, a subroutine is called to send the command and store thereplies on an appropriate stack. Control transfers to step 1301 of FIG.13.

At step 1302, a message in format 1130 is broadcast from monitor 124with command code 1131 set to the value of the variable C (initially 0)and argument 1132 set to the value of the variable G (initially thegroup of interest).

At step 1304, for each reply slot wherein a reply is detected, a valueindicating a reply was detected may be stored on a stack identified froman array of stacks indexed by the variable RS. By providing an array ofstacks, interrogation proceeds according to a tree search algorithmwherein at each node up to 1000 replies are cataloged. Each stacktherefore corresponds to one of the nodes traversed in a modifieddepth-first tree search. In step 1304, information associated with eachreply may also be stored on the appropriate stack. Such information mayinclude: (a) the reply slot number; (b) signal amplitude samples 417;(c) frequency domain results of one or more fast Fourier transforms ofsamples 417; (d) one or more signal properties; and (e) a figure ofmerit as discussed above with reference to FIGS. 3 and 4. Forefficiency, pointers to such information may be stacked instead.

At step 1306, control returns from the send/stack subroutine back tostep 1206 of FIG. 12.

At step 1206, variable C is set to 3 as an initial condition for thesubroutine called in step 1208.

At step 1208, a subroutine is called to list member identificationnumbers. This subroutine is a recursive subroutine which accomplishesthe modified depth-first tree search as discussed above beginning fromthe current value of variable RS initially set to 1. Control transfersfrom step 1208 to step 1401 of FIG. 14.

At step 1402, it is determined whether variable RS is at a maximumvalue. Variable RS indicates a level code as described with reference toFIG. 8. Command 1004 having already been accomplished at step 1204, RSwill proceed from the value 1 to a maximum value of 4 corresponding tocommands 1004 through 1007 discussed with reference to FIG. 10. Havingreceived control from step 1208, the test at step 1402 will fail andcontrol will pass to step 1406.

At step 1406, variable G is assigned the value (or values) popped fromthe top of STACK [RS]. In an alternate implementation the access code tobe used for the value G is obtained from a table look-up operation(e.g., code conversion mapping). For example, the number of a reply slotis used as an index into an array and the value from the array isassigned to G for use as an access code. In an implementation whereinsubgroups are not addressed in strict order of depth, a level code maybe used as part of the index and the array value may identify a suitablelevel code in addition to the access code.

At step 1408, it is determined whether process 1208 has proceeding tothe end of STACK [RS]. If so control passes to step 1402 for a returnfrom this particular call of the list members recursive subroutine. Ifnot, control passes to step 1410.

At step 1410, the validity of the value (or values) of variable G isdetermined. This validity test may proceed in a manner similar todetermining whether a particular reply represents a candidate frequencyas described at step 506 and 510 above. This analysis may includeanalysis of time domain results, frequency domain results, signalproperties, and figures of merit, provided that sufficient informationhas been stored on STACK [RS]. Time domain analysis may compare thesignal received or properties (e.g., rise time, decay time, envelopeshape, or relative time of peak amplitude) with expected values orproperties in accordance with the Q of tank 204 and power limitingcharacteristics discussed below with reference to signal REPLY of FIG.16. If it is determined that variable G does not represent a validtransceiver, control passes back to step 1406 for obtaining anothervalue from STACK [RS]. Otherwise, control passes from step 1410 to step1412.

At step 1412, variables C and RS are each incremented. By incrementingthe value of variable RS, results will be stored on a new (empty) stack.By incrementing the value of variable C, preparations are made totransmit a command at the next level.

At step 1414, send command and stack replies subroutine 1204 is calledfrom the context of the current level and current command set at step1412. Upon return from step 1306, control transfers to step 1416. In thefirst call to subroutine 1204 from routine 512, message format 1120, orpreferably 1130 may be used. In subsequent calls, from step 1414,message format 1130 alone or preferably prefixed by any suitable numberof message formats 1140 may be used. Prefix message formats 1140, whenused, assure proper access state bit prerequisites are met by contentsof respective arguments 1142. Prerequisites may have been reset by lossof operative power or by reset as discussed below.

At step 1416, a recursive call is made to the list members subroutinewithin the context of the current value of variable RS. Controltransfers to step 1401 and upon completion returns from step 1420.

At step 1417, variables C and RS are decremented to restore the contextof the current execution of recursive subroutine list members 1208.Processing in the loop consisting of steps 1406 through 1417 continuesuntil all replies have been considered from STACK [RS]. When all replieshave been considered, control passes from step 1408 to step 1420 and areturn to a prior call of list members subroutine 1208 is effected.During execution of list members subroutine 1208 at the deepest level(i.e., the highest value of variable RS), control is transferred fromstep 1402 to step 1418.

At step 1418, the respective reply slot numbers of the replies receivedin response to the command sent at step 1302 are appended to an arrayherein called the member list. As a result of the tree search algorithm,values from STACK [RS] are appended from time to time until the listmembers subroutine has reached the end of the stack at the initial levelof the tree (i.e., level equals 1 and RS equals 1). When the tree hasbeen fully searched, the return from step 1420 passes control to step1210 of FIG. 12.

At step 1210, interrogate subroutine 512 of FIG. 12 returns control tomethod 500 at step 512. Processing continues at step 512 to selectanother monitor transmit frequency for interrogation from array MTFI asindexed by loop variable N until loop variable N exceeds the value B.For each frequency, interrogate subroutine 512 beginning at step 1201 iscalled for an appropriate tree search. At step 1418, redundantidentification numbers may be appended to the member list. Consequently,step 1418 may include a test to forego appending a transceiveridentification number to the member list unless it is not already on themember list. Upon completion of interrogation at each monitor transmitfrequency for interrogation, control transfers to step 514.

At step 514, the contents of the member list array may be reported tohost computer 122. This reporting function may be accomplished (oraccompanied) by a printout, display, alarm, etc., at monitor 124 asdiscussed above. Further, the function of reporting identifiedtransceiver identification numbers may be accomplished by suitable filestorage or conventional communication between programs operative on hostcomputer 122 and/or monitors 124, 126.

At step 516, host computer 122 and/or monitor 124 may initiate anycommand sequence including, for example, command 1008 for commands 914through 920 as discussed above. Following completion of all individualcommand/reply sessions (if any), control passes to step 518 where method500 may repeat beginning at step 502 for continuous monitoring.

The determination of frequencies to be used for interrogation asdiscussed above provides a list of frequencies (e.g., array MTFI) priorto any interrogation. In an alternate method, interrogation may proceedimmediately upon detection of a response believed to be transmitted by atransceiver. Further, a command/reply session may be performedimmediately upon determining a transceiver. The internal iteration loopsin each of steps 502 through 516 in such an alternate method arereplaced with appropriate controls on the major iteration loop of step518.

In subsequent iterations of method 500, steps 502 through 508 may beomitted when no additional transceivers are expected to have recentlyentered communication range. Selected interrogation frequencies of arrayMTFI may be omitted when use provided no identification not alreadyknown by use of other interrogation frequencies. Step 512 may then beperformed with a minimum of redundancy to decrease time spentinterrogating. Further, when subsequent interrogations reveal no newtransceiver identifications, steps 510 through 514 may be omitted andstep 516 executed repeatedly for a list of specific transceiveridentifications. For a system that monitors continued presence oftransceivers without command/reply sessions, step 512 may be repeatedwith specific (non-redundant) frequencies to receive acknowledgementfrom each transceiver by fully addressing the transceiver via its knowncomplete identification. Monitoring presence of a known populationgenerally is accomplished in less time than interrogation of an unknownpopulation. Conversely, to the extent that an unknown populationpredictably includes transceivers having identifications in known groups(or subgroups to any level), the time spent performing an interrogationmay be reduced by addressing and communicating with members of suchknown groups (or sub-groups to any level). Likewise if a group (orsub-group) is known not to be present (or communication is not desiredwith transceivers of such group or subgroup), interrogation may befashioned to ignore replies or avoid facilitating replies fromtransceivers of such a group (or sub-group).

Step 512 may be omitted for object identification systems where merepresence of one or more objects is all that is desired to be monitored,for example, setting an alarm on detection of any object carried througha passage. Monitoring of objects in the presence of other objects may beaccomplished in an alternative implementation. For example, an alarm maybe set on detection of any object through a passage, except whenaccompanied by detection of another predetermined object (e.g., atransceiver in a badge of an authorized person).

Step 508 may be omitted when step 506 provides sufficient resolution ofone or more frequencies.

A method for improving reception of a reply signal during interrogationor data communication includes the steps of: (a) transmitting a carriersignal at a first frequency; (b) sampling a reply signal, (c)identifying one or more frequency components not expected to be part ofa proper reply signal; and (d) programming a filter to attenuate suchfrequency component(s). The transmitted carrier may be at a tankresonant frequency, a stack resonant frequency, or a frequency suitablefor use with a tank circuit loosely coupled to a stack. Sampling andidentifying frequency components may be accomplished in any mannerincluding further time domain signal processing and/or frequency domainsignal processing, as discussed above. The filter may include a digitalfilter, programmable element network, or a programmable active filter.The filter characteristic may include a low-pass, band-pass, notch,comb, or hi-pass transfer function. Transmitting and sampling may occurduring a reply slot.

A method for improving the accuracy of an interrogation scenarioincludes the steps of: (a) determining a first series of amplitudesamples of a reply signal; (b) comparing the first series to a secondseries of amplitude values expected for a resonant circuit response; and(c) proceeding in the protocol of the interrogation scenario inaccordance with whether the extent of comparison exceeded a thresholdvalue.

A transceiver, according to various aspects of the present invention,includes any circuit for performing the process discussed above withreference to FIG. 8. For example, a transceiver 201 capable ofperforming the command set of FIG. 10 using the message formats of FIG.11 may receive and send data using a combination of off-on keying (OOK)and duty cycle modulation.

The functions of rectifier 206, receiver 208, transmitter 210, and statemachine 212 may be better understood from a timing description ofsignals used in transceiver 201. During an interrogation scenario,several messages may be received by a transceiver. Each message to whicha reply is expected from any transceiver constitutes a query. Aninterrogation scenario may include several queries. For example, FIG. 15presents signal TANK as it would appear across lines 217 and 219 (i.e.,the difference of signals N1 and N2). Portions of signal TANK correspondto portions of an interrogation format 1500 which includes start portion1593, preamble portion 1594, message type portion 1595, message portion1596, and reply slots portion 1597. Signal TANK is rectified byrectifier 206 to provide DC voltage V+ which is used to power allcircuitry of transceiver 201. Signal TANK is demodulated by receiver 208to provide signal DEMOD on line 214. And, signal TANK includes bysuperposition the output of transmitter 210 in response to modulationsignal MOD on line 216. From time T1502 to time T1504, transceiver 201receives unmodulated carrier on signal TANK. The period of time fromtime T1502 to time T1504 represents a START signal 1593 as discussedabove with reference to process 802. The duration of the START signalshould be sufficient to energize rectifier circuit 206 for the provisionof continuous power to transceiver 201 for the duration of operationrequired by the interrogation protocol.

Following the START signal, signal TANK exhibits a series of periods of50% duty cycle modulation sufficient for establishing proper timingsignals for use within transceiver circuitry 201. For example, signalCELL CLK is derived from signal DEMOD on line 214 and signal RX CLK isderived to have active edges in the midpoint between the active edges ofsignal CELL CLK. Signal CELL CLK represents a cell clock which marks byits active edges the trailing edge of each cell used for communicationof one data bit. From time T1504 to time T1506 no carrier is beingreceived. From time T1506 to time T1510 carrier is being received. Thispattern of off/on keying is repeated for the entire preamble portion1594 until time T1516. The length of preamble portion 1594 should besufficient for generating all timing signals for use in transceivercircuitry 201.

Received clock signal RX CLK has an active edge in the middle of eachdata communication cell for discriminating between cells conveying alogic “0” and cells containing a logic “1”. Cell content clocked bysignal RX CLK is illustrated as signal RXD conveying a “010” pattern formessage type portion 1595.

The logic “0” of signal RXD is derived from a cell containing modulationin only the latter portion of the cell duration. For example, no carrieris received from time T1516 to time T1520; however, carrier is receivedfrom time T1520 to time T1522. The duration from time T1520 to timeT1522 divided by the cell duration (from time T1516 to time T1522)represents a duty cycle of from 10% to 45%, preferably 40%. The activeedge of signal RX CLK occurs while signal DEMOD is low at time T1518from which signal RXD is determined as a logic “0”. In contrast, thenext cell beginning at time T1522 and extending to time T1530 includes aportion from time T1522 to time T1524 where no carrier is being receivedand a portion from time T1524 to time T1530 during which carrier isbeing received. The duration from time T1524 to time T1530 divided bythe cell duration (from time T1522 to time T1530) represents a dutycycle (different from the duty cycle of the cell from T1516 to T1522) offrom 55% to 90%, preferably 60%. The active edge of signal RX CLK occurswhile signal DEMOND is high at time T1526 from which signal RXD isdetermined as a logic “1”. The following cell extends to time T1534 andexhibits another logic “0”.

Message type portion 1595 of interrogation format 1500 extends from timeT1516 to time T1534. Following message type portion 1595, messageportion 1596 extends from time T1534 to time T1550. During messageportion 1596, signal TANK and signal DEMOD convey data using off/onkeyed modulation, preferably with 40% and 60% duty cycle modulation. Ina variation, each bit of message type portion 1595 is sent as twocomplementary bits in sequence (e.g., a dibit) to facilitate a form ofredundancy for message validity testing. Similarly, command and/orargument portions of any message format 1100, 1110, 1120, 1130, or 1140may be sent as dibits.

Signal CELL CLK and signal RX CLK continue through message portion 1596(not shown for clarity). From time T1550 to time T1580 reply slots 1597are distinguished by signal TANK. Reply slots 1597 include a reply slotfor each reply. The duration of a reply slot is equivalent to one periodof signal CELL CLK. For example, from time T1550 to time T1554 nocarrier is received; however, from time T1554 to time T1558 carrier isreceived. The signal received from time T1554 to time T1558 (andanalogous times in other reply slots) serves several functionsincluding: to maintain power supplied by rectifier circuit, to mark aboundary between adjacent reply slots, to define a duration (e.g., acell clock period) for synchronizing other clock signals (e.g., a signal8 times the cell clock frequency), to identify the beginning of anoffset into the reply slot for signal detection (e.g., placement of theactive edge of signal RCV CLK), and to identify the beginning of anoffset into the reply slot for transmitting a reply signal. By markingthe boundary of a reply slot with carrier for a predetermined portion ofsignal CELL CLK (e.g., 10% to 90% preferably 40% to 60%, most preferablyabout 50%), signal CELL CLK can remain synchronized to boundaries of allreply slots. In an alternate implementation where transfer of powerduring reply slots is not required, signal CELL CLK may be synchronizedwith preamble portion 1594 and monitor 124 may transmit nothing duringreply slots 1597.

During the reply slot from time T1550 to time T1558 no reply isindicated. The reply slot from time T1558 to time T1566, however,includes a reply during the portion of the slot where signal MODindicates transmitter 210 is providing modulation. Signal MOD enablestransmitting from time T1560 to time T1562, that is, during a time whenno carrier is being provided by monitor 124. As will be explained ingreater detail with reference to FIG. 16, the duration of signal of MODoverlaps a portion of the carrier transmitted by monitor 124.

Any number of reply slots may be used. When 1,000 reply slots aredefined, signals may have the durations as described in Table 4. Thesignals in Table 4 correspond to a message format 1140 followedimmediately by a message format 1130. The reply slot used for replyingto this series of message formats is the reply slot associated with thecommand and argument portions immediately preceding the reply slotsportion, regardless of the number of preceding message formats. Ofcourse, the same argument values may be used redundantly for assuringproper reception. In this example, argument 1142 may be the same asargument 1132.

TABLE 4 Message Format Approximate Periods of Portion Duration CellClock Reference START 3,200 μsec 32 or T1502-T1504 Continuous Carriermore PREAMBLE 800 μsec 8 T1504-T1516 50% OOK TYPE 300 μsec 3 T1516-T153440%/60% OOK COMMAND 300 μsec 3 1141 40%/60% OOK ARGUMENT 1,000 μsec 10 1142 40%/60% OOK PAD 200 μsec 2 1143 No Carrier SEPARATOR 800 μsec 81146 50% OOK PREAMBLE AND 1,100 μsec 11  — TYPE COMMAND, 1,500 μsec 15 1131, 1132, 1133 ARGUMENT, and 40%/60% OOK PAD REPLY SLOTS 100,000 μsec1,000    1135, T1550-T1580 TOTAL: 109.2 msec 1,092   

The timing diagram of FIG. 16 illustrates the use of additional clocksignals for deriving signal RX CLK and signal MOD. Signal TANK is shownin one cell consisting of a first portion from time T1602 to time T1610where no carrier is being received and a second portion from time T1610to time T1616 where continuous carrier is being received. Signal DEMODis illustrated with a transition corresponding to 50% duty cyclemodulation. Signal RX CLK provides an active edge (in the center of thecell) corresponding to the rising edge of signal DEMOD as illustrated.Signal DEMOD when conveying a logic “1” would have a rising edge at timeT1608 providing sufficient set-up time prior to the active of signal RXCLK. When signal DEMOD is conveying a logic “0”, the rising edge ofsignal DEMOD is delayed until time T1614 providing sufficient hold timefollowing the active edge of signal RX CLK.

Signal MOD may be formed by signal Q2 from time T1606 to time T1610. Itis preferred to extend the duration of signal MOD beyond time T1610 sothat modulation provided by transmitter 210 overlaps transmission ofcarrier by monitor 124. By overlapping the transmission of signals bymonitor 124 and transmitter 210, transmitter 240 in an adjacenttransceiver in unlikely to confuse a lack of modulation between thefalling edge of signal MOD for example, at time T1610, with the boundaryof the cell which occurs at time T1616. In this way, each transceivermay accurately recognize a cell boundary by the falling edge of signalDEMOD and maintain synchronism of clock signals including signal CELLCLK.

Signal REPLY of FIG. 16 illustrates that portion of signal TANK thatwould be superimposed on signal TANK when transceiver 201 istransmitting a reply in response to signal MOD. From time T1606 to timeT1612, the amplitude of signal REPLY depends on the Q of tank circuit204 and available power for transmitting. From time T1606 to time T1609,amplitude depends largely on Q. From time T1609 to time T1612, amplitudedecreases as power available for transmitting decreases (thoughsufficient power may remain for logic functions).

Transceiver 201 may be constructed on a substrate as an integratedcircuit. The cost of integrated circuit fabrication for a circuit oflimited complexity (e.g., transceiver 201) is adversely affected by thearea of the substrate dedicated to pads for connection of the integratedcircuit to external devices. A preferred set of pads for integratedcircuit interface signals is described in Table 5. Using conventionalvoltage discrimination or alternate mode control circuitry, pads may beused for multiple signals and other pads omitted. For example, pad 2 mayalso be used for signal FUSE PROG; pad 6 may also be used for signalFUSE DATA; and pad 7 may also be used for signal FUSE CLK.

TABLE 5 Signal Pad Name Function Reference 1 VSS Ground 1721 2 FRC Usedto connect on external energy storage 1718 capacitor to ground 3 V+ Usedto connect a filter capacitor to ground 1717 4 N1 Antenna coilconnection 217 5 N2 Antenna coil connection 219 6 VXC Used to connect afilter capacitor to ground 1817 7 VJ Used to connect a filter capacitorto ground 2103 8 FUSE Serial data for programming memory 2214 2310 DATA9 FUSE Serial clock for programming memory 2214 2312 CLK 10 FUSE Enablesserial data to blow fuses 2315 PROG

Rectifier circuit 206 may include any conventional circuitry fordeveloping a direct current voltage from a received carrier signal. Forexample, rectifier 206 of FIG. 17 includes bridge rectifier 1700 acrosslines 217 and 219, energy storage capacitor C1710, series regulatorcircuit 1712, and circuit 1716 for determining when the developedvoltage is of sufficient magnitude for transceiver operation. Rectifiercircuit 1700 includes diodes D1702, D1704, D1706, and D1708 in aconventional full wave bridge arrangement. Tank circuit 204 (includingantenna 202 and capacitor 1703) is connected across the center of bridge1700. Full-wave rectified capacitance signal FRC on line 1718 may becarried to an external connection for additional capacitance to ground.Regulator circuit 1712 receives signal FRC on line 1718 and presents ina conventional manner signal V+ having a suitable voltage magnitude online 1717. Comparator 1716 compares signal V+ on line 1717 with theoutput of a conventional voltage reference circuit 1714 (e.g., a bandgap reference circuit, zener diode, etc.). Comparator 1716 providessignal VOK when the voltage on line 1717 exceeds the output of voltagereference 1714. Signal VOK enables transceiver operation. Rectifiercircuit 206 may receive sufficient power for transceiver operation whenmonitor 124 transmits at the resonant frequency of tank 206, the stackresonant frequency as discussed above, or any frequency and power levelthat accommodates the transfer function of tank 206 (including antenna202).

Receiver 208 may include any conventional receiver circuitry. Particularadvantages are obtained in system 100 by receiver circuitry 208 of FIG.18 which includes detector 1808, flip-flop 1812, phase locked loop 1814,and gate logic 1824. Receiver 208 may be operated at the resonantfrequency of tank 206, the stack resonant frequency as discussed above,or any frequency and power level that accommodates the transfer functionof tank 206 (including antenna 202).

Detector 1801 includes a fall wave rectifier, a filter, and, a Schmidttrigger inverter. Signal N1 on line 217 passes through diode D1802 toline 1809 and is shunted to ground by filter capacitor C1806 and filterresistor R1808. Likewise, signal N2 on line 219 passes through diodeD1804 and connects to line 1809. Line 1809 provides a signal across theshunt filter to Schmidt trigger inverter 1810. Inverter 1810 providessignal DEMOD on line 1823. Signal DEMOD clocks T flip-flop 1812 toprovide a 50% duty cycle signal on line 1811.

Phase locked loop 1814 includes phase frequency detector 1816, voltagecontrolled oscillator (VCO) 1818, and counter 1820. VCO 1818 operates at160 KHz to provide oscillating signal VCQ on line 1819. Signal VCQ isdivided by counter 1820 to provide 80 KHz, 40 KHz, 20 KHz, and 10 KHz.The 50% duty cycle signal on line 1811 is compared with 10 KHz signalCELL CLK on line 1821 by phase frequency detector 1816 to providevoltage control signal VXC on line 1817.

Gate logic 1824 provides signals RX CLK on line 1827 and signal TX GATEon line 1829 in a conventional manner in accordance with the timingdiagram of FIG. 16.

Receiver 208, in an alternate configuration, may include detector 1902of FIG. 19 in place of detector 1801. Detector 1902 includes inverter1904, switch transistor 1905, and a filter having capacitor C1906 andresistor R1908. Inverter 1904 receives signal FRC on line 1718 fromrectifier 206. Switch transistor 1905 cooperates with capacitor C1906 ina manner similar to a charge pump (e.g., an integrator) to providesignal DEMOD on line 1823.

Transmitter 210 may be any conventional transmitter circuit. Particularadvantages are obtained in system 100 using a transmitter circuit ofFIG. 20 which includes analog switch 2002, amplifier 2006, and tankcircuit 204. Tank circuit 204 forms the only resonant circuit intransmitter 210. Tank circuit 204, therefore, governs the frequency oftransmitter 210. Any magnetic coupling in antenna 202 may affect theresonant frequency of tank 204 and thereby affect the transmittedfrequency provided by transmitter 210. Transmitter 210 may includeeither a Colpitts or Hartley oscillator design. For example, transmitter210 of FIG. 20 includes capacitor C2004, amplifier 2006, capacitorC2008, and bridge capacitors C2010 and C2012. Bridge capacitors togethercorrespond to capacitance C1703 described above. Capacitors C2004 andC2008 provide AC coupling and DC blocking in a conventional manner.Analog switch 2002 receives signal MOD on line 216. When signal MOD isasserted, a feedback signal on line 219 is coupled to amplifier 2006 tocomplete the closed loop oscillator.

In an alternate transmitter, the frequency to be transmitted isdetermined in part by the frequency previously received. For example,transmitter 210 of FIG. 21 includes phase locked loop 2100 and isolationcircuit 2112. Phase locked loop 2110 includes phase frequency detector2102, sample hold circuit 2106, and voltage controlled oscillator 2110.VCO 2110 operates at 5 MHz to provide signal OSC on line 2111 to phasefrequency detector 2102. Signal N1 on line 217 is also coupled to phasefrequency detector 2102. Phase frequency detector 2102 responds to aphase difference between signal N1 and signal OSC to provide signal VJon line 2103. Sample hold circuit 2106 responds to signal DEMOD on line1823 to hold the value of signal VJ when signal N1 is not beingreceived. Sample hold circuit 2106 provides signal VK on line 2107 tocontrol the oscillator frequency of VCO 2110.

Reply frequencies for transmitters discussed above with reference toFIGS. 20 and 21 are described in Table 6. The transmitter of FIG. 21 ispreferred for implementations involving stacks.

TABLE 6 Transmitter Type Reply Frequency Colpitts Oscillator Tankresonant frequency when transceiver operates in isolation; any stackresonant frequency when within a stack; between tank resonant frequencyand stack resonant frequency when loosely coupled to a stack (e.g., onan end or in non-coplanar orientation). Phase Locked Loop As driven bycarrier from monitor 124 (e.g., at an isolated tank resonant frequency,a stack resonant frequency, or any other desirable frequency. Thecarrier frequency may be selected for any one or more of the followingreasons: (a) to avoid the carrier being masked by interfering frequencycomponents (e.g., of antenna system 121, or of signal 193); (b) to avoidthe reply being masked by interfering frequency components (e.g., ofantenna system 121 or of signal 193); (c) to assure adequate powertransfer to enable one or more transceivers; and (d) to prevent adequatepower transfer or adequate received signal quality from enabling one ormore transceivers not currently of interest. For example, if a stackresonant frequency has been detected at 4.3 MHz, the monitor maytransmit at a predetermined offset (e.g., less 500 KHz) from 4.3 MHz tointerrogate a transceiver loosely coupled to the stack (e.g., at an endof a linear stack) whether or not a response (e.g., a ring signal) wasdetected at that offset.

By sampling received signal N1 while carrier is being provided bymonitor 124, and holding the frequency received to establish thefrequency to be used for transmitting, transmitter 210 of FIG. 21provides a transmitted signal at a frequency better suited tocommunication with monitor 124. Transmitter 210 may have a transmitfrequency as specified by monitor 124 as opposed to a frequency asdetermined by tank 204. Operation of transmitter 210 as discussed aboveis particularly advantageous for objects 107 and 112 each located at anend of stack 116. Although the resonant frequency of coupled tanks ofobjects 108 thorough 111 may be detected by monitor 124 for the purposeof providing sufficient power and accurate data communication at aparticular selected carrier frequency, the same carrier frequency maynot couple sufficient power into objects 107 and 112 or provide reliablecommunication due to the weaker coupling between the tank circuits ofobjects 107 and 108, for example, in as much as the tank circuit ofobject 107 is not between two other similar objects.

State machine 212 may include any conventional state machine circuitryfor performing the functions described above. For example, state machine212 may include circuitry as described in FIG. 22 which includes synclogic 2202, shift register 2204, latch 2206, comparator 2208, accessstate logic 2210, memory 2214, and counter 2224. These devices cooperateto provide interrogation commands 1004 through 1007 as described above.Additional logic may be added to access state logic 2210 to supportcommands 1000 through 1003 using conventional techniques. State machine2102 may be expanded to perform command 1008 corresponding to commands914 through 920 as discussed above. In such an expanded configurationstate machine further includes multiplexer 2212, sensor 2216, analog todigital converter 2218, multiplexer 2220, multiplexer 2222, shiftregister 2240, and multiplexer 2228.

Shift register 2202 receives signal DEMOD on line 1823 as clocked bysignal RX CLK on line 1827. The content of shift register 2202 isconsidered a valid message when the message type portion 1595 of theparallel data output of shift register 2202 corresponds to apredetermined message type code. For example, type “010” may be used asillustrated in FIG. 15 for signal RXD beginning at time T1518. Type“010” is used herein for all commands described above with reference toFIG. 10. Other message type codes may be used; or, additional messagetype codes may be used in an expanded set of commands as described abovewith reference to FIG. 9. Shift register 2202 provides in parallel dataformat on bus 2203 the message type code, an access code, and acorresponding level code. The message type code is provided to synclogic 2204. The access code (e.g., argument 1132 of message format 1130)is provided to comparator 2208. The level code (e.g., command 1131 ofmessage format 1130) is provided to latch 2206, access state logic 2210,multiplexer 2212, and multiplexer 2220. Shift register 2202 may includea holding register to hold the output codes for processing until asubsequently received message has arrived in fall. A subsequent messageis deemed to have arrived in full when a valid message type code followsa START signal 1593 and preamble 1594 as illustrated in FIG. 15.

A signal discriminator includes any circuit that derives mode controlsignals (e.g., message type, load, preset, etc.) and timing signals(e.g., resets, and clocks) from a composite signal. For example,receiver 208 cooperates with sync logic 2204 to provide a discriminatorthat derives signals from received messages. For example, receiver 208derives signal CELL CLK on line 1821 and sync logic 2204 receives amessage type code on bus 2203 from shift register 2202 and receivessignal CELL CLK on line 1821 and provides various reset signals. Synclogic 2204 may, in addition, receive and generate further clock signalsof higher frequency than signal CELL CLK on line 1821. Sync logic 2204establishes, inter alia, the initial conditions for latch 2206, accessstate logic 2210, and counter 2224. Sync logic 2204 detects a power-oncondition and establishes initial conditions in response thereto. Synclogic 2204 clears latch 2206 and clears all access state bits B0-B3 inaccess state logic 2210 using signal SRST on line 2223. Sync logic 2204provides signal CRST on line 2201 to clear counter 2224 as an initialcondition. Sync logic 2204 also provides signal CEN to comparator 2208to enable comparison at a time determined, for example, from time T1516corresponding to the beginning of message type portion 1595 of aninterrogation format illustrated in FIG. 15. Time T1516 can bedetermined from a predetermined number of active edges on signal CELLCLK according to a suitable protocol.

Latch 2206 includes an addressable access state flip-flop for eachaccess state bit B0-B3. Signal LEVEL is used as an address to select aflip-flop to be set. A selected flip-flop is set by the cooperation ofsignal CEN on line 2231 and signal D on line 2235. The leading edge ofsignal CEN provides a clock and signal D establishes the state of theaddressed flip-flop. Signal D is provided on line 2235 from access statelogic 2210 in accordance with: (a) the access state provided on line2207 by latch 2206, and (b) signal LEVEL on line 2203. By allowingaccess state logic to determine signal D under various conditions, theaddressed flip-flop in latch 2206 may be set as discussed above withreference to commands 1004 through 1007 or may be left unaffected as forcommands 1000 through 1003, as discussed above. Latch 2206 provides theoutput of each flip-flop as signal ACCESS STATE on line 2207 to addressstate logic 2210.

Access state logic 2210 receives signal ACCESS STATE on line 2207 fromlatch 2206 and receives signal LEVEL on line 2203 from shift register2202. Based on these inputs, access state logic 2210 may provide asubstitute memory address signal on line 2209 with suitable controlsignals 2211 to effect selection by multiplexer 2212 of an appropriateaddress on line 2217 to be used for recalling an access code from memory2214. In an alternate implementation where signal LEVEL is used directlyas a memory address to memory 2214, multiplexer 2212 may be omitted withappropriate simplifications to access state logic 2210. In such animplementation, address input 2217 of memory 2214 is supplied by shiftregister 2202 on bus 2203 to provide signal LEVEL as the address. Accessstate logic 2210 provides read-write control to memory 2214 on line 2213as signal R/W. Access state logic 2210 also provides control signals2211 to multiplexer 2222 for the selection of data to be provided on bus2225 as signal MDATA.

Multiplexer 2222 provides bus 2225 to comparator 2208, counter 2224, andshift register 2240. Signal MDATA conveys a stored access code tocompactor 2208, or memory contents or sensor data to counter 2222 andshift register 2240.

When enabled by signal CEN on line 2231, comparator 2208 providesresults of comparison on signals 2205 to access state logic 2210. Forexample, when an access code on bus 2203 exactly matches a stored accesscode provided from memory 2214 on bus 2225, an A=B output of comparator2208 is asserted and provided to access state logic 2210. When signalCEN enables comparison and the access code on line 2203 is not exactlyequal to the access code on bus 2225, an A≠B output is asserted bycomparator 2208 and provided to access state logic 2210. In a preferredconfiguration, access state logic 2210 responds to an A≠B signal bydriving signal SRST on line 2233, thereby resetting latch 2206 to itsinitial condition, and notifying sync logic 2204 to provide any furtherreset or initial conditions as may be suitable. In effect, regardless ofthe sequence in which multiple access codes with various levels arepresented for comparison, if any one such access code is not exactlyequal to the corresponding access code recalled from memory 2214, statemachine 212 reverts to its initial condition and awaits a subsequentSTART signal. Consequently, an addressed transceiver will enter a resetstate (and may enter a power-off state) to avoid transmitting when notproperly addressed at a subsequent level. Control signals 2211 providedby access state logic 2210 control all aspects of the operation of statemachine 212 in a conventional manner. One such control signal, signal OSon line 2215, directs multiplexer 2228 to provide signal MOD inaccordance with output selection signal OS, as discussed below.

Memory 2214 may include any conventional data storage technology, ormultiple such technologies, in any combination. Memory 2214 may beorganized to provide memory contents on line 2223 in parallel format, asshown, or in serial format in an alternate architecture. In such analternate architecture, state machine 212 may include a serialcomparator in place of the parallel comparator 2208. Memory 2214provides on line 2223 a 10-bit access code in parallel with a 10-bitreply slot number. The reply slot number may be transferred throughmultiplexer 2222 and loaded into counter 2224. Memory 2214 providesstorage for any number of (access code, reply slot) pairs. In apreferred implementation, 4 such pairs provide a unique transceiveridentification and 4 additional pairs provide an alternateidentification or support for alternate interrogation protocols. Forexample, commands 1000 through 1003 may have different respectiveargument values, one for each command. The GID used in command 1004 maybe identical to the GID used in command 1000. These four GID “standard”values may be stored in many (e.g., all) transceivers for use in aparticular installation of system 100. Knowledge of one or more of thesefour “standard” GID values by monitor 124 (or host 122) facilitiesinterrogation in any sequence of commands 1004-1007 when prerequisitesare not used or are modified accordingly.

Counter 2224, when clocked by signal CELL CLK on line 1821, providessignal ZM on line 2227 when the reply slot number is decremented tozero.

Multiplexer 2228 provides signal MOD on line 216 in response to the ANDcombination of signal TX GATE on line 1829 and signal ZM on line 2227 toenable transmission of a reply acknowledgment in the reply slotassociated with the access code provided simultaneously on memory outputline 2223.

To support commands of the type described in FIG. 9, for example,commands 914 through 920, for example command 1008 of FIG. 10, statemachine 212 may load any or all contents of memory 2214 into shiftregister 2240 by appropriate operation of multiplexer 2222 by accessstate logic 2210 via control signals 2211. When loaded as describedabove, shift register 2240 responds to signal CELL CLK on line 1821 asenabled by counter 2224 output on line 2231 to provide signal QM on line2229.

Access state logic 2210 may provide signal OS on line 2215 tomultiplexer 2228 to provide three reply message formats. First, whensignal OS selects multiplexer input A on line 2227, the proper timingfor a reply in a prescribed reply slot (e.g., reply slots 1125 or 1135)is provided by signal MOD on line 216. When signal OS on line 2215 isasserted to enable multiplexer input B, signal QM on line 2229 in ANDcombination with signal TX GATE on line 1829 determines the state ofmodulation signal MOD on line 216. Signal MOD on line 216 consequentlyconveys the contents of shift register 2240 ad seriatim. Counter 2224may be operated in conjunction with shift register 2240 usingconventional logic for one of two functions: (a) providing a fixednumber of bits from shift register 2240 ad seriatim on line 216 assignal MOD in a second reply message format (e.g., reply bits 1114); or(b) providing one bit from shift register 2240 in each occurrence ofreply slots portion 1597 until the entire contents of shift register2240 has been provided in a manner corresponding to conventional timedomain multiplexing in a third reply message format (e.g., reply slots1125 or 1135).

Sensor 2216 represents any electronic transducer including sensors ofthe type described above with reference to sensors 160 and 162. Sensor2216 provides an analog signal to analog to digital converter (ADC)2218. ADC 2218 provides sensor data signal SDATA on lines 2219 tomultiplexer 2220. Multiplexer 2220, operated by control signals 2211,permits the selection of either received data signal RDATA on bus 2203from shift register 2202 or sensor data signal SDATA on line 2219 to beeither: (a) stored in memory 2214 via bus 2221; or (b) provided throughmultiplexer 2222 to either counter 2224 or shift register 2240. Whenprovided to counter 2224, sensor data, for example a 10-bit value, mayoperate as a reply slot number as described above for the provision of areply signal in one reply slot. When provided to shift register 2240,selected data may be used to provide signal MOD on line 216 in any ofthe reply message formats described above.

Received data signal RDATA, when used to form modulation signal MOD online 216, provides the capability for a transceiver to echo data asreceived for accomplishing testing a single transceiver. Tests mayinclude (a) testing data communication reliability in a laboratoryenvironment; and (b) testing transceiver reliability in the presence ofexternal factors including, for example, variation in facilityenvironment, variation in the strength and frequency of interferingsources, and variation in the number and proximity of similartransceivers in a laboratory or installation environment.

In response to a suitable command, access state logic 2210 may invoke awrite operation by asserting signal R/W on line 2213 to memory 2214.Data to be written into memory may be provided by shift register 2202 assignal RDATA on bus 2203 through multiplexer 2220, or may be provided bysensor 2216 through multiplexer 2220. Data written into memory mayinclude original (or revised) access code and slot number for one ormore values of signal LEVEL. Write memory operations may be used tofacilitate code hopping as discussed above.

Portions of state machine 212 may be omitted to reduce powerconsumption, to reduce the cost of manufacture of transceiver 201, orwhen one or more functions are not desired for an installation of system100. For example, sensor 2216, ADC 2218, and multiplexer 2220 may beomitted when transceivers are not used for sensing the environmentsurrounding a transceiver. Further, multiplexer 2222 may be omitted whentest functions described above are not desired. Shift register 2240 andmultiplexer 2228 may be omitted when message format 1130 or 1120 issufficient for a reply and message format 1110 is not desired. Memory2214 may be read only in which case signal R/W on line 2213 may beomitted with concomitant simplifications to access stage logic 2210.

Memory 2214 may include read-write memory organized as conventionalrandom access memory (RAM) or as shift register memory. Further, theread-only portions of memory 2214 may include any combination of ROM,PROM, EPROM, E²PROM, and fuse programmable memory. Particular advantagesare obtained in transceiver 201 by use of a circuit for fuseprogrammable memory. For example circuit 2300 of FIG. 23 includes shiftregister 2302, decoder 2304, and an array of programmable fusesexemplified by programmable fuse circuit 2314 and tri-state driver 2316for each memory bit. Circuit 2300 accepts on line 2310 serial signalFUSE DATA conveying binary data to be stored in memory. Shift register2302 is clocked by signal FUSE CLK on 2312 until all data to be storedin memory has been received. Upon assertion of signal FUSE PROG on line2315, each fuse element in respective fuse circuit 2314 issimultaneously programmed in accordance with the parallel output ofshift register 2302. The fuse element in fuse circuit 2314 may be anyconventional fuse element including a diode, a zener diode, apolysilicon fuse, or a metal element. After programming, any group ofprogrammed fuses 2322 may be asserted on bus 2223 when signal ADDR online 2217 drives decoder 2304 to provide a suitable tri-state bufferenable signal for example, as on line 2327. The enable signal on line2327 enables tri-state buffers 2324 to provide memory output data on bus2223 as signal Q. Fuse circuits 2314 may be grouped in any suitablemanner to form any number of data output bytes or words in response tocorresponding addresses defined for signal ADDR.

According to various aspects of the present invention, power sufficientfor transmitting in one reply slot is obtained primarily from thecarrier received during a START portion of the message format. When atransceiver provides no more than one reply per START signal, the REPLYsignal may decay during transmitting. Rapid decay assures transmittingwill not continue into a succeeding reply slot; facilitates applicationof maximum power during transmitting prior to the onset of decay; andpermits exhaustion of power during transmitting to inevitably result ina full reset of the access state (e.g., when signal VOK is no longerasserted).

In an implementation including battery power for transceiver circuits,the beneficial operating features discussed in the preceding paragraphmay be obtained by transferring (for a limited duration) power from thebattery to a capacitor which provides limited power as discussed above.

Monitor 124 may include any computer controlled transmitter/receiver forconducting a suitable interrogation protocol and communication asdiscussed above. In addition, a monitor of the present invention maycooperate with various sensors 160, provide various controls 164, andcooperate with various antennas organized as an antenna system 120. Forexample, monitor 124 as shown in the functional block diagram of FIG. 24includes central processing unit (CPU) 2402, memory 2404, andconventional data communication bus 2406. Data bus 2406 couples CPU 2402and memory 2404 for the conventional execution of stored programs inmemory 2404 by CPU 2402. Bus 2406, in addition, provides datacommunication between CPU 2402 and functional blocks including: computernetwork control 2408, event detectors 2410, output register 2411,antenna network control 2412, receivers 2416 and 2418, digital signalprocessor (DSP) 2420, transmitters 2424 and 2426, and programmablefrequency source (PFS) 2422. Transmitters 2424 and 2426 providetransmitted signals to coupler 2414; and, coupler 2414 provides receivedradio frequency signals to receivers 2416 and 2418. By providing twofunctionally equivalent receivers and two functionally equivalenttransmitters together with a coupler, monitor 124 may simultaneouslytransmit on two frequencies and receive on two other independent bandssimultaneously. To that end, PFS 2422 provides signal ProgrammableFrequency Source Output (PFSO) on line 2423 to each transmitter 2424 and2426. Signal PFSO may be provided to each transmitter on separate linesat different frequencies. Receivers 2416 and 2418 each receivingrespectively signal RF on line 2417 and signal RFN on line 2419, mayprovide samples of received signals in digital format on bus 2421 to DSP2420. CPU 2402 may control DSP 2420 to prescribe: (a) operation with oneor both receivers 2416 and 2418; (b) a time to begin processing samplesfrom bus 2421; (c) a duration for sampling; (d) configuration parametersfor selecting a method for digital signal processing; (e) a method andformat in which DSP 2420 provides results; and (f) the destination forthe results, i.e., whether to CPU 2402, to memory 2404 for furtherprocessing by CPU 2402, or to computer network control 2408 for transferto host computer 122.

DSP 2420 may perform digital signal processing including amplitudeaveraging, calculation of power, digital filtering, peak detection, timedomain edge enhancement, phase analysis, frequency analysis,transformation (e.g., fast Fourier transformation), correlation,superposition, curve-fitting, and power spectral density calculation.

Memory 2404 provides storage for programs and data used primarily by CPU2402 and DSP 2420. Memory 2404 may include data structures, arrays,stacks, and combinations thereof for storage of signal properties asdiscussed above. Memory 2404 (or host 122) may also include indicia ofgroup identification and sub-group identification (to any level) for usein interrogation scenarios. These indicia may exclude (or not include)access codes reserved for use in other independent implementations ofsystem 100. For example access code ranges may be specifiedalgorithmically or as one or more lists wherein not all access codevalues or combinations of values are made available for use duringinterrogation.

Computer network control 2408 may include any conventional interface forcoupling data bus 2406 to host computer 122. For example, computernetwork control 2408 may include a conventional ethernet interface. Bus128 provided by computer network control 2408 may conform to anycomputer network standards, for example, any conventionaltelecommunications network standard or a standard used in communicationvia the Internet and the World Wide Web. Computer network control 2408may include one or more additional processors for maintaining, forexample, a TCP/IP stack, or performing any suitable protocol. Computernetwork control 2408 (and/or CPU 2402) may communicate with hostcomputer 122 using a command language as described in Table 7. Eachcommand includes an ASCII character to identify the command followed byargument values. Operating frequencies may be identified in variouscommand/answer sessions by integers called bins. For example, anoperating range from 1.9 MHz to 8.038 MHz may be divided into 1024 binswherein the frequency corresponding to a given bin integer is computedfrom the expression: F(bin)=bin*6 KHz+1900 KHz.

TABLE 7 Command/Answer Description N <Antenna Node> <Antenna Address>Direct the set up and selection of antennas for a <Antenna Mode> <Gain><Frequency> monitor to use in a specified mode (e.g., transmit, receive,test). Set antenna node RF channel operating parameters. Specify afrequency for antenna node tuner to use to tune the selected antenna(s).No response. An acknowledge answer may be used. G <Squelch delay><Squelch width> <Receive Specify Monitor receiver operating parametersdelay> <DSP Start-up Delay> <DSP Sample and analog switch settings.Squelch delay Count> <DSP Mode> <Ch. A Mode> <Ch. A facilitatesbeginning squelch at a zero crossing Signal Source> <Ch. A Gain> <Ch. Aof energy on the antenna(s) to be squelched. Filtering> <Ch. A Clock><Ch. A Output> Squelch width corresponds to duration D434. {etc. for Ch.B} Receive delay may direct beginning receiving on or after the T416(e.g., at times A or B as discussed above). DSP sample count conveys thenumber of samples to be taken (e.g. 32 μsec window for FFT calculation).DSP mode may be as defined by an integrated circuit DSP (e.g., T1320marketed by Texas Instruments). Ch. A/B mode may direct transmit,receive, or both (loop back) Ch. A/B Signal Source may select samesource for two receive channels. Ch. A/B clock source may directfrequency and phase (e.g., 0°, +90°) for signal SC. Ch. A/B output maydirect which of several detectors is/are used. No answer. An Acknowledgeanswer may be used. C {Ch. A antenna arguments} {Ch. B antenna Directsthe set up and selection of antennas for arguments} <Start frequency><End each (e.g., A and B) receiver in the Monitor frequency> <Frequencystepping> with arguments similar to N command. Requests amplituderesults (e.g., received amplitude or received power) from each receiverin a specified range of frequencies (i.e., bins) by specifying the binnumber range to be reported (e.g., from bin 123 to bin 885). May specifyan increment between bins (e.g., report every fifth bin). {<Ch. ADetector Output at Bin p>} . . . Reports up to 1024 amplitude values foreach {<Ch. B Detector Output at Bin q>} . . . channel (e.g., p = 0 to1023; and q = 0 to 1023). May substitute DSP output when FFT results aredesired. O {<Header> <Level> <Access Code>}. . . Interrogate a group,subgroup, or particular transceiver. The list Header may define asequence and number of arguments (e.g., level and access code) in the Ocommand. One or more N command arguments may precede the list. {<Ch. Aat Reply Slot p>} . . . {<Ch. B at An integer for each of two receivechannels Reply Slot q>} . . . (e.g., A and B) is provided for each of1024 reply slots (e.g., p = 0 to 1023; q = 0 to 1023). Result depends onG and N command values for antenna, receiver, and DSP operating modes.The integer may represent any of the following: (a) whether amplitude(or power) exceeded a threshold value; (b) a magnitude of a detectedamplitude (or power); (c) a magnitude of a frequency component (e.g., asprovided by an FFT calculation). In an expanded version, the answer mayinclude a list of integers for each integer in (b) for time domainsampling and (c) for frequency domain results.

Receivers 2416 and 2418 may be any conventional receivers. Particularadvantages are obtained in system 100 by use of receiver circuitry 2416of FIG. 25 which includes preamplifier 2502, diode detector 2504,synchronous detector 2506, analog switch 2508, filters 2510, ADC 2512,first-in-first-out (FIFO) register 2514, and control registers 2526.Preamplifier 2502 receives signal RF on line 2417 and providesamplification and automatic gain control (AGC). The gain and frequencyresponse characteristics of preamplifier 2502 are prescribed in aconventional manner by signals 2509 from control registers 2526 asspecified by CPU 2402. Preamplifier 2502 provides signal RFW on line2503 to one or more detectors.

Diode detector 2504 receives signal RFW on line 2503 and providesdemodulated signal DX on line 2505. Any conventional diode detector maybe used. Diode detector 2504 represents a wide-band detector preferredfor detecting transceivers in a stack, especially transceivers having atransmitter of the type described with reference to FIG. 20. Particularadvantages in system 100 are obtained by using diode detector 2504 ofFIG. 26.

Synchronous detector 2506 receives signal RFW on line 2503 and providesdemodulated signal SX on line 2507. Any synchronous detector circuitrymay be used to provide the demodulation function including, for example,a conventional tracking filter circuit. Synchronous detector 2506represents a narrow-band detector.

Analog switch 2508 under direction of control registers 2526 selects oneor more detector output signals, for example, signal DX on line 2505and/or signal SX on line 2507 and provides a demodulated signal (e.g., asuperposition) to filters 2510.

Filters 2510 may implement any filtering transfer functions (e.g.,low-pass, band-pass, high-pass, and notch) as directed by signals online 2511 from control registers 2526 as directed by CPU 2402. Theoutput of filters 2510 is converted to digital samples by ADC 2512. Suchsamples are stored in FIFO 2514 and provided to CPU 2402 and/or DSP2420. ADC 2512 may include any conventional analog to digital convertercircuit. When receiving a 5 MHz response or reply signal, samples 417may be acquired at 40 MHz to provide sufficient resolution for signalanalysis, as discussed above. In an alternate implementation of receiver2416, signal RFW is coupled to ADC 2512 and detection and filtering areaccomplished by CPU 2402, DSP 2420, or by host computer 122 using, forexample, conventional digital technologies.

Diode detector 2504 of FIG. 26 includes inverting amplifier 2602,non-inverting amplifier 2604, transistors Q2606 and Q2608, capacitorC2610, resistor R2612, and output buffer 2614. Amplifiers 2602 and 2604receive signal RFW on line 2503 and provide base drive signals totransistors Q2606 and Q2608. Transistors Q2606 and Q2608 rectify theradio frequency content of signal RFW. Capacitor C2610 and resistorR2612 cooperate as a filter to receive rectified signals fromtransistors Q2606 and Q2608 and provide the filtered wave form to outputbuffer 2614. Output buffer 2614 provides signal DX on line 2505 in aconventional manner.

Particular advantages are obtained in system 100 by using a synchronousdetector of the type described in FIG. 27. Synchronous detector 2506 ofFIG. 27 includes inverting amplifier 2702, non-inverting amplifier 2704,analog switch 2706, filter 2708, and programmable oscillator 2710.Amplifiers 2702 and 2704 receive signal RFW on line 2503 and providebuffered signals to analog switch 2706.

Analog switch 2706 selects the output of amplifier 2702 for the outputof amplifier 2704 under the direction of signal SC on line 2705 fromprogrammable oscillator 2710. Programmable oscillator 2710 operates at afrequency, phase, and duty cycle prescribed by CPU 2402 through controlregisters 2526 received on line 2501 by programmable oscillator 2710.Phase may be relative to zero crossings detected in a conventionalmanner from signal RFW. Programmable oscillator 2710 may include aconventional synchronizer circuit for receiving signal RFW and providingsignal SC in a phase relationship to signal RFW as directed by signalsof control registers 2526. The output of analog switch 2706 may includeharmonics of the switching frequency of signal SC. Filter 2708 receivesthe output of analog switch 2706 and attenuates unwanted frequencycomponents. Filter 2708 may include any conventional filter circuit, forexample, a low-pass, notch, band-pass, comb, etc. Filter 2708 providessignal SX on line 2507.

Receivers 2416 and 2418 may be operated, each with a synchronous(narrow-band) detector. The received signals may be received on one ormore suitable antennas or delayed to provide a 90° phase differencebetween otherwise identical signals prior to detection. When onesynchronous detector is operated at the same frequency as the other yetwith a 90° phase shift in clocking signal SC, the detected amplitudescorrespond to conventional I and Q signals for phase detection andsignal analysis based on phase, as discussed above.

Transmitters 2424 and 2426 may include any conventional transmittercircuitry. Particular advantages are obtained in system 100 by usingtransmitter circuitry 2424 of FIG. 28 which includes shift register2802, counter 2804, multiplexer 2806, duty cycle modulator 2808, controllogic 2812, and output gate 2810. Data bus 2406 from CPU 2402 providestransmitter 2424 with information to be transmitted as well asconfiguration parameters for control logic 2812. Information to betransmitted is loaded into shift register 2802 in accordance withsuitable control signals 2830 provided by control logic 2812. Thecontents of shift register 2802 may conform to message formats describedabove with reference to FIG. 11 and FIG. 15. In both cases the replyslots portion of message formats 1120 and 1130 may be provided byoperation of counter 2804, loaded by suitable control signals 2830. Forexample counter 2804 may be loaded with the value 1,000 to provide 1,000reply slots. Multiplexer 2806 receives serial data shifted out of shiftregister 2802 on line 2803 and receives counter output Q0 on line 2805.Control logic 2812 provides a selection signal via control signals 2830to control multiplexer 2806 so as to provide the content of shiftregister 2802 followed by the number of reply slots directed by theinitial count of counter 2804. Operation of multiplexer 2806, therefore,provides on line 2807 a signal in a message format to be transmitted.

Duty cycle modular 2808 may respond to control codes of control signals2830 from control logic 2812 and the signal on line 2807 to providemodulated signal TXG on line 2809. Off/on keying and duty cyclemodulation are provided in a manner consistent with the contents ofTable 8.

TABLE 8 Control Code Resulting Modulation 00 No carrier. 01 40%modulation for transmitting a “0” data bit. 10 60% modulation fortransmitting a “1” data bit. 11 Uninterrupted, unmodulated carrier.

Transmitted signal XD on line 2425 is provided by the AND combination ofsignal TXG on line 2809 (defining a transmit gate) and signal PFSO online 2423. Signal PFSO defines an unmodulated carrier frequency asprogrammed by CPU 2402 on line 2423.

Antenna system 120 may be controlled in accordance with a physicaldistribution of antennas so as to support multiple antennas at each ofone or more nodes connected by an antenna bus. Each antenna node mayprovide for coupling one or more antennas to the transmitter and/orreceiver portions of monitor 124 in any convenient manner. Antennas maybe coupled for balanced or unbalanced use in receiving or transmitting.When multiple antennas are used for transmitting, antennas may be drivenin different phases. When multiple antennas are used for receiving,received signals may be delayed for synchronization or provided in adesired phase relationship. Because different antennas of antenna system120 may have different radiation (or reception) patterns operation of atransmitter with one or more antennas and/or a receiver with one or moreantennas provides advantages for communication with transceiversregardless of transceiver orientation and proximity to othertransceivers as discussed above. Antenna system 120 provides areconfigurable multi-antenna system with a tuning capability for eachantenna. In addition to tuning each antenna, antenna system 120 has theability to squelch any antenna used for transmitting and couple thesquelched antenna to a receiver for immediate reuse as a receivingantenna. Antenna system 120 provides multiple transceiver channels ineach antenna node with the capability of routing signals from onechannel into another for signal processing.

The functions described above for antenna system 120 may be provided byone or more antenna nodes cooperating on an antenna bus. Particularadvantages for system 100 are obtained by using the antenna node circuitdescribed in a functional block diagram of FIG. 29. Antenna node 140 asdescribed in FIG. 129 includes CPU 2902 and memory 2904 coupled togetherby data bus 2906 for program execution. Antenna node 140 furtherincludes antenna network interface 2908, input register 2909, outputregister 2910, coupler 2912, coupler 2914, a plurality of antennas 150(including antenna 2916), and a plurality of transceiver channels 2918.

CPU 2902 receives commands and information and provides status usingdata communication on antenna bus 132, coupled by antenna networkcontrol 2412 to CPU 2402. CPU 2402 of monitor 124 provides commandsinterpreted by CPU 2902 for functions described in Table 9.

TABLE 9 Command/Answer Description A <Antenna Node Address> Read statusof input register(s) (e.g., manual switches), status of outputregister(s) (e.g., current matrix switch settings, squelch settings,tuner settings, RF channel settings, feedback settings, any memoryaddress (e.g., antenna node software version, tuner calibration date,number of installed antennas, etc.). <Antenna Node Address><Answer DataSeveral different commands may be used to Length><Answer Data><Checksum>obtain status in part. B <Antenna Node Address><Settings Data Set outputregister(s) contents to specify Length><Settings Data><Checksum> antennaconfiguration, antenna(s) coupling to transceiver channel(s), squelchsettings for each channel, tuner settings for each channel, feedbacksettings for each channel. No answer. An Acknowledge answer may be used.C <Antenna Node Address><Configuration Set configuration data in memoryincluding Data Length><Configuration Data> antenna node address, antennaaddresses, <Checksum> function(s) to be executed on manual switchclosure, table of tuning settings (e.g., relay closures vs. frequency),table of antenna settings (e.g., relay closures vs. frequency orconfiguration identifier), any memory address (e.g., tuner calibrationdate, number of installed antennas, etc.). No answer. Several differentcommands may be used to specify configuration in part. An Acknowledgeanswer may be used.

Memory 2904 provides storage for programs executed by CPU 2902, storagefor configuration information for other functional blocks of antennasnode 140, and tuning parameters used in transceive channels 2918. Thisinformation may be organized in memory 2904 in any conventional datastorage format.

Antenna network interface 2908 provides data transfer and control amongantenna bus 132 data bus 2906, and coupler 2914. Antenna networkinterface 2908 may provide serial to parallel and/or parallel to serialdata format conversion for transferring signals between serial antennabus 132 and parallel data bus 2906. Antenna network interface 2908 maybuffer received signals from transceive channels 2918 to receivers 2416and 2418 of monitor 124. Further, antenna network interface may receivemodulated carrier signals from transmitters 2424 and 2426 in monitor 124and provide buffered signals for transceive channels 2918. Receivedsignals and modulated carrier signals pass between antenna networkinterface 2908 and coupler 2914 on line 2905.

Input register 2909 monitors the state of switch 2907 and communicates aswitch closure event via data bus 2906 to CPU 2902. Switch 2907 mayprovide any manual data entry function. Switch 2907 is representative ofany number of switches, for example, toggle switches or a data entrykeyboard. In a preferred configuration, switch 2907 when closed, directsCPU 2902 to provide one or more test and/or measurement functions. Suchfunctions include identifying a test mode to CPU 2402 of monitor 124 viaan appropriate data communication message via antenna network interface2908. Because antenna node 140 may be packaged and located at a locationremote from host computer 122 and/or monitor 124, the convenientlocation of a manual switch 2907 for test and/or measurement functionssimplifies installation and maintenance of system 100 including theinstallation and maintenance of antenna system 120.

Output register 2910 receives data from data bus 2906, stores such data,and maintains output signals in accordance with stored data. Signalsprovided by output register 2910 direct operation of coupler 2912 andtransceive channels 2918. Output register signals on line 2913 controlcoupler 2012 9 (e.g., configuration and matrix switch operations).Squelch command signals on line 2921 direct antenna squelching functionsof squelch circuit 2920. Tuning signals on line 2923 direct tuningfunctions of tuner 2922. Finally, digital signals on line 2927 controloperation of transceiver channels 2924 (e.g., specifying preamplifiergain, automatic gain control, and filter transfer functions). Outputregister signals on lines 2913, 2921, 2923, and 2927 are binary digitalsignals and may be used in common across multiple transceive channels2918, or additional digital signals may be provided by output register2910 for each transceive channel.

Coupler 2912 may include any conventional circuit for coupling anantenna to an RF channel. For example, coupler 2912 provides a matrixswitch for the coupling of any antenna of antennas 150 (for example,antenna 2916) to one or more transceive channels 2918. In like manner,any transceive channel, for example 2924, may be coupled to one or moreantennas 150 through coupler 2912. Coupler 2912 provides abi-directional coupling for both received and transmitted signals andsupports multiple received and transmitted signals simultaneously.Coupler 2912 may also provide appropriate switching to select antennaelements of an individual antenna 2916 of antennas 150. For example, oneor more of lines 2911 and 2915 may be coupled to one or more lines 2925and 2935 to implement: (a) phased array transmission or reception; (b)use of antennas (or elements) in sequence; (c) scanning whileinterrogating or transferring data; or (d) providing operative power onantenna(s) different from the antenna(s) used for interrogation or datatransfer. Coupler 2912 couples antenna elements (e.g., of antenna 2916)for use with one or more transceive channels 2918 in accordance withsignals on line 2913 received from output register 2910. Antenna elementselection as discussed above may be performed for any one or moreantennas of antennas 150.

Coupler 2914 may include any conventional RF switching circuitry forcoupling and buffering modulated carrier signals and received signalsbetween antenna network interface 2908 and one or more transceivechannels 2918. For example, when antenna bus 132 provides one modulatedcarrier signal for transmission, antenna network interface 2908 mayprovide the modulated carrier on signal line 2905 to coupler 2914.Coupler 2914 may couple the modulated carrier signal via one or moresignals TRI1 2951 through TRIN 2955 to one or more transceive channels2918. In addition, coupler 2914 may buffer any received signal (e.g.,TRI1 through TRIN) to provide any one or more feedback signals TRC1 2953through TRCN 2957 signals TRC1 through TRCN from coupler 2914 permit afirst transceive channel 2924 to provide its output signal TRI1, forexample, in accordance with: (a) antenna signals 2925 received fromcoupler 2912, and (b) signals received through any one or more othertransceive channels 2918, for example, RF front channel 2934. Coupler2914, therefore, provides for the combination of received signals fromone or more RF channels to be provided on signal line 2905 to antennanetwork interface 2908. Coupler 2914 enables a single RF channel (e.g.,2924) to combine a channel signal (e.g., 2925) with a signal from one ormore other RF channels (e.g., TRC1 . . . TRCN) and provide the resultingreceived signal (e.g., TRI1) on line 2905 to antenna network interface2908.

Transceive channels 2918 include one or more parallel circuits forperforming, inter alia, antenna tuning and squelch functions. Eachtransceive channel is a functional equivalent of other transceivechannels to provide similar (yet configurable) functions on each ofseveral channels. Each transceive channel includes an RF channelcircuit, a tuner, and a squelch circuit.

RF channel circuit 2924 may provide transmit signal buffering andreceived signal filtering and amplification in any conventional manner.Particular advantages in system 100 are obtained using RF channelcircuit 2924 of FIG. 30. RF channel 2924 of FIG. 30 includes amplifier3002, analog switch 3004, filters 3006, programmable preamplifier 3008,differential amplifier 3010, and analog switch 3012. For a modulatedcarrier signal to be transmitted on an antenna 150, RF channel circuit2924 receives signal TRI on line 2929, provides buffering andamplification via amplifier 3002, and passes the buffered signal throughanalog switch 3004 as signal TRA on line 2925 to coupler 2912. For asignal received from coupler 2912, signal TRA on line 2925 passesthrough analog switch 3004 to filters 3006. Filters 3006 provide anyconventional filtering function (e.g., low-pass, band-pass, notch, andhigh-pass analog or digital filtering). When received signal TRAincludes OOK modulation, filters 3006 may include time domain signalprocessing functions, for example, Schmidt triggering and/or edgeenhancement functions. Programmable preamplifier 3008 responds tocontrol signals 2927 from output register 2910 to provide a transferfunction with programmable gain at various frequencies, (e.g., automaticgain control).

Differential amplifier 3010 receives the output signal from preamplifier3008 and may receive a signal TRC1 on line 2953 via analog switch 3012.Analog switch 3012 is controlled from output register 2910 via signals2927. When analog switch 3012 allows passage of signal TRC1 todifferential amplifier 3010, differential amplifier 3010 may perform ananalog subtraction to provide a difference signal TRI1 on line 2951. Theanalog subtraction may provide additional common mode rejection; or, mayprovide an enhanced signal for receiving a reply from a transceiver 201when, for example, more than one antenna 150 is used for the receptionof the reply signal. Differential amplifier 3010 may includeprogrammable phase shift circuits for enhancing the common moderejection or signal enhancement capability under the direction ofsignals from output register 2910. Phase correction may be desirablewhen antennas of different configurations or different orientationssupply signals to differential amplifier 3010.

Tuner 2922 matches the impedance of an antenna (e.g., antenna 2916) toan RF channel circuit 2924. The effects of coupler 2912 and antennaelement selection performed by coupler 2912 are accounted for byoperation of tuner 2922. Tuner 2922 may include any conventional tuningcircuit. Tuner 2922 preferably includes impedance matching elements thatare selectively introduced between an RF channel circuit and an antennain responsive to signals from output register 2910. Memory 2904 mayinclude data and methods for determining suitable control signals fortuner 2922.

Memory 2904 (or memory 2404, or memory in host computer 122) includes anarray of values for output register 2910, each value including a bit tocontrol closure of each of several switches in tuner 2922. The array isindexed by an integer corresponding to a desired operating frequency(e.g., a bin number as discussed above). Values for such an array may bedetermined according to an antenna test method. An antenna test methodaccording to various aspects of the present invention includes the stepsof: (a) direct a transceiver channel (e.g., including tuner 2922) to beused for both transmitting (e.g., from transmitter 2424) a test signaland for receiving (e.g., using receiver 2416), the test signal having asuitable amplitude and test frequency throughout the test method; (b)direct use of a narrow band detector (e.g., synchronous detector 2506 ofFIG. 27); (c) select and direct a switch closure combination and observea detector output; (d) compare the detector output to a maximum observeddetector output; (e) if the detector output exceeds the maximum detectoroutput, update the maximum observed detector output to match thedetector output and note the switch closure combination corresponding tothe detector output; (f) repeat steps (c) through (e) until all switchcombinations have been selected; (g) record the switch closurecombination corresponding to the maximum detector output in an array forthe tested antenna (or combination of antennas) indexed by the testfrequency. After testing each installed antenna (separately or incombination(s) with other installed antennas) at one or more testfrequencies, results of several tests may be stored in an integratedarray that includes for each frequency a recommended antenna (orcombination of antennas) and a recommended tuner switch closurecombination. The integrated array may be stored in volatile ornonvolatile memory in host computer 122, memory 2404, or memory 2904.

Particular advantages are obtained in system 100 by using tuner circuit2922 of FIG. 31. Tuner circuit 2922 of FIG. 31 includes one or moreshunt circuits 3100 spanning signal lines 2925 that pass through tuner2922 between RF channel circuit 2924 and coupler 2912. Each shuntcircuit includes, respectively, an analog switch and a capacitor. Forexample, analog switch 3101 is controlled by an output signal fromoutput register 2910. When closed, analog switch 3101 connects capacitorC3102 across lines 2925 to increase the capacitive load. In like manner,analog switch 3103 may connect capacitor C3104; and, analog switch 3129may connect capacitor C3130. Capacitors C3102, C3104, and C3130 may havevalues in a binary sequence, for example, as conventionally used in adigital to analog converter circuit.

Memory 2904 may include a method for operation of squelch circuit 2920to perform a suitable squelch function as discussed above. Antennanetwork interface 2908 may provide a mechanism for analyzing the phaseof a signal to be transmitted and provide such phase information on databus 2906 to CPU 2902. Phase information may be indicated by a suitableinterrupt corresponding to a zero crossing. Squelch command signals onlines 2921 provided by output register 2910 may be clocked throughoutput register 2910 in accordance with phase information as discussedabove, when signal processing latency of CPU 2902 (e.g., interruptlatency) does not provide a squelch command signal in reliable closeproximity to a zero crossing of a modulated carrier signal to betransmitted. CPU 2902 may provide a command signal to output register2910 that accounts for variations in RF channels 2918 and variations inmodulated carrier signals so as to operate squelch circuit 2920 in anefficient manner. The squelch function is considered efficient whenenergy on antenna elements is quickly dissipated in close proximity to azero crossing of the phase of a signal to be transmitted. The squelchfunction should account for energy stored in all related circuitryincluding distributed capacitances of, for example, coupler 2912, tuner2922, and RF channel 2924.

Squelch circuit 2920 may include any conventional squelch circuitry.Particular advantages are obtained in system 100 by using squelchcircuit 2920 of FIG. 32 which includes inverter 3201, gated sourcepositive (GSP) 3202, filtered rectifier positive (FRP) 3204, gatedsource negative (GSN) 3206, filtered rectifier negative (FRN) 3208, FRP3244, GSP 3244, FRN 3248, and GSN 3246.

GSP 3202 includes switch transistor Q3210 having a base connected toanalog switch 3212 and to analog switch 3214. The collector oftransistor Q3210 is coupled to FRP 3204. In operation, signal SQ* drivesanalog switch 3214 to couple a −9 volt supply through resistor R3216 tosink current from the base of transistor Q3210 turning transistor Q3210on, and providing current from a +5V source at the emitter of transistorQ3210 through the collector to FRP 3204. In an opposite phase, signal SQdrives analog switch 3212 to couple a +9V supply to the base oftransistor Q3210, quickly turning transistor Q3210 off. GSN 3206 is ofanalogous structure for operation with an opposite polarity fortransistor Q3211.

FRP 3204 includes a series filter consisting of resistor R3230 and theparallel combination of resistor R3232 and capacitor C3234. The outputof the series filter feeds a node between a pair of diodes D3236 andD3238. Diode D3238 is forward biased by the current provided throughresistor R3230 and resistor R3232. Diode 3236 clamps leg 3282 of signals2925 to a voltage that is one diode drop above ground; the diode dropbeing provided by diode D3238. FRN 3208 is of identical structure as FRP3204 except that diodes D3237 and D3239 are in reverse polarityorientation. In operation squelch circuit 2920 of FIG. 32 clamps legs3281 and 3282 alternatively to plus and minus current sources forquickly extinguishing any potential difference between legs 3281 and3282. In operation, an antenna may be squelched using circuit 2920 ofFIG. 32 in a manner sufficient for use with a receiver in less thanone-half cycle of a transmitted carrier signal (e.g., in about 35 μsec).

Antenna bus 132 may be any serial or parallel bus for the control ofantenna system 120 by monitor 124. Antenna system 120 may be organizedas a bus, a daisy-chain, a star, or a hierarchical combination ofsubnetworks. Particular advantages are obtained in system 100 by usingan antenna bus 132 comprising four differential signals physicallyarranged as a network controlled by monitor 124. For example, antennanetwork interface 2908 of FIG. 33 includes interface buffers 3302, 3304,3314 and 3316, signal selector 3306, shift register 3308, transmitbuffer 3310, receive buffer 3312, shift register 3320, outputmultiplexer 3322, and control register 3324. Antenna bus 132 includesany suitable number of the signals described in Table 10.

TABLE 10 Signal Name Signal Description TC Transmit control. Signal TCcontrols signal selector 3306 that couples signal TD to either shiftregister 3308 (as a command to CPU 2902) or to transmit buffer 3310 (forproviding signal TRI on line 2905 to coupler 2914). TD Transmit data.Signal TD provides a serial command which may include an ASCII commandcharacter followed by one or more bytes of argument values to be usedwith the command; or, a modulated signal to be routed by coupler 2914for transmission. RC Receive control. Signal RC controls outputmultiplexer 3322 for the selection of either signal TRI from coupler2914 through receive buffer; or, data from CPU 2902 converted fromparallel to serial format by shift register 3320. The signal resultingfrom selection drives interface buffer 3316 to provide signal RD. RDReceive Data. Signal RD is provided only when CPU 2902 determines aproper address from a received command (e.g., matching a predeterminedaddress). Signal RD may provide a serial response (via shift register3320) from CPU 2902 to a command (received via shift register 3308).Signal RD may, alternatively, respond to transceive channel output fromcoupler 2914 (via receive buffer 3312) to provide a received signal tomonitor 124.

When signals TC and RC are asserted, data communication in serial onlines TD and RD provide information flow between CPU 2902 and CPU 2402of monitor 124. When signals TC and RC are not asserted, signals TD andRD provide transmit and receive signals, respectively, from one or moretransceive channels to one or more transmitters or receivers in monitor124. Antenna bus 132 uses differential line drivers for all signals sothat antenna nodes may be physically distributed a considerable distanceapart for the convenience of locating antennas 150 and 152. Monitor 124(via antenna network control 2412) supplies signals TC, TD, and RC toall antenna node interfaces 2908 of respective antenna nodes 140, 142simultaneously. Data communication via signals TC, TD, and RC mayinclude any conventional protocol to coordinate nonconflicting use ofeach shared signal line (e.g., lines for signals RD). For example,signal TD may include a command followed by an address. Each CPU 2902may compare the received address from shift register 3308 with apredetermined address so as to enable line driver 3316 via controlregister 3324 at a time dictated by the protocol and the result ofaddress comparison so as to obtain data communication to monitor 124without interference from other antenna network interfaces in otherantenna nodes.

Control of antenna node 140 by monitor 124 may be obtained using anyconventional command set and command syntax, for example, the commandsdiscussed above with reference to Table 9.

Antennas 150 may include one or more antennas having various geometriesfor the detection of reply signals from one or more transceivers ofsystem 100. Planar antennas in a variety of configurations may be used.For example, antennas defined in FIGS. 34 and 35 provide particularadvantages in system 100. These planar antennas may be supported by oneor more antenna nodes 140, 142 in any convenient combination as desired.Passage 3500 includes walls 3506 and 3507, top 3504, and base 3505arranged over ground plane 3501. Although not drawn to scale, passage3500 preferably has a square aspect ratio for the opening through whichobjects may pass. Passage 3500 has been found to provide suitableperformance when constructed as a passageway for personnel (includingportions of a building, e.g., floor, wall, or ceiling of a hallway) andwhen constructed as a passageway for carriers of objects (having anopening approximately three feet square, i.e., about one meter square).Smaller passages may be used for tabletop instrumentation.

A reference coordinate system having an origin 3510 serves to define theplane of each planar antenna. Angle alpha (α) is measured in the XYplane from the X axis. Angle beta (β) is measured in the XZ plane fromthe X axis. Angle gamma (γ) is measured in the YZ plane from the Y axis.

As a practical matter, an antenna having more than one turn, may notexist in one plane. However, the planar antennas described in FIG. 34may be manufactured to approximate the antenna pattern that would beproduced by a theoretical planar antenna. Alternatively, antennas atsimilar planar angles may be formed (or loops arranged) along an axisperpendicular to the plane (e.g., helical).

Antenna 3401 is constructed in the plane defined by points A, B, C, D,i.e., in a plane parallel to the XZ plane at the opening of the passagefiuthest on the Y axis from origin 3510. Antenna 3402 is parallel toantenna 3401 yet closer to origin 3510. Movement of a transceiver alongan axis through the passage parallel to the y axis may be determined byexamination of the time when the peak reply signal strength is receivedfrom each of antennas 3401 and 3402. Antenna 3403 is again parallel tothe XZ plane and in addition exists at the mid-point of the passage(e.g., each point J, K, L, exists at the mid-point of a segment NB, OC,PD parallel to the Y axis). Antenna 3404 may be arranged at an angleα=45° when passage 3500 is essentially cubic in geometry. Similarly,antenna 3405 may be perpendicular to antenna 3404 when passage 3500 isessentially cubic. Antenna 3406 is oriented in a plane having anglesα=135° and γ=135° and is of the type described in related patentapplication Ser. No. 09/233,755, cited above. Antenna 3407 has anorientation complimentary to antenna 3406. Antenna 3408 lies in a planeparallel to the ground plane 3501. Antenna 3409 and antenna 3410 areparallel to the YZ plane and may be constructed in sides 3506 and 3507,respectively.

Transceive channel circuitry, particularly squelch circuit 2920 shouldbe located as specified in the Table for optimum performance (minimalgeneration of out-of-band noise). Points T,U, and V bisect segments LK,HG, and DC respectively. Point S bisects segment PK.

A passage including antennas 3402, 3403, 3406, 3407, 3408, and 3409 ispreferred for an object identification system wherein objects 102through 112 pass through the passage for identification and datatransfer. Other combinations of the antenna orientations discussed abovemay be used for economy, reliability, or to enhance particular systemperformance.

Any antenna of antennas 150 may be constructed of multiple loops as aplanar antenna. Particular advantages are obtained in system 100 byusing an antenna of the type described in FIG. 36. Antenna 3600 includesthree loops and terminals 3601, 3602, 3603 referenced to a commonterminal 3611. Loops may be formed of any conductor including a shieldedconductor for limiting E-field radiation while sending or receivingmagnetic field radiation. In addition, antenna 2916 includes Q modifyingcircuit 3604. Q modifying circuit 3604 includes diode D3612, diodeD3614, and resistor R3616, all connected in parallel terminal 3610 toterminal 3611. In operation, a transmit signal, for example, signal TRAon line 2925 through coupler 2912, may be imposed across two terminals:a first selected from the set consisting of terminals 3601, 3602, and3603; and a second selected from the set consisting of 3610 and 3611.When terminal 3610 is used, a transmit signal of suitable magnitude mayforward bias diodes D3612 and D3614 to shunt resistor R3616. Arelatively high Q antenna circuit results. On the other hand, a signalreceived by antenna 2916 having a signal magnitude insufficient toforward bias diodes D3612 and D3614 will pass through resistor R3616. Arelatively low Q antenna circuit results. A lower Q antenna is typicallycharacterized by a wider band sensitivity than a higher Q antenna. Whentransmitting energy intended to power one or more transceivers, a higherQ antenna is preferred.

When objects 102 through 112 are to be interrogated while passingthrough a passage of the type described or discussed above withreference to FIG. 35, interrogation and data communication reliabilitymay be enhanced by arranging objects 102 through 112 in one or moretransportation carriers. A transportation carrier, according to variousaspects of the present invention, includes one or more resonant antennacircuits for focusing transmitted and received energy. Carrier 3700 ofFIG. 37 is exemplary of any structure in which objects of the typedescribed above may be located for convenient interrogation and datacommunication. A carrier having any geometry may be used for extendingor shaping the antenna sensitivity pattern of the antenna of an object,for example, antenna 202 of object 104 or FIG. 2. For example,transportation carrier 3700 includes side walls 3702 and 3704, and base3706. In addition, carrier 3700 includes antenna circuit 3708 comprisinga loop conductor and series capacitor C3710. Antenna circuit 3708, byvirtue of the value of capacitor C3710, has a resonant frequencyselected to enhance energy transferred to an object and/or communicationbetween monitor 124 and an object. In a preferred configuration, antennacircuit 3708 is arranged with a relatively low Q and at a resonantfrequency substantially different from frequencies which may be used forinterrogation and data communication. When monitor 124 provides a scansignal or subscan signal of the type discussed with reference to FIG. 4,the ring signal associated with antenna circuit 3708 may be easilyidentified as discussed above so that interrogation at the resonantfrequency of antenna 3708 may be avoided.

Carrier 3700 may include a second antenna circuit 3716 constructed in amanner similar to antenna circuit 3708 with a series capacitance C3714.Antenna circuits 3708 and 3716 may be coupled in any convenient manner(e.g., interdigitated loops, overlapping portions) arranging a portionof each loop in close proximity for magnetic field or electric fieldcoupling.

Memory, as discussed above, may include any apparatus for data storage(e.g., semiconductor circuits, circuits of discrete components, andmagnetic and/or optical media.

The foregoing description discusses preferred embodiments of the presentinvention which may be changed or modified without departing from thescope of the present invention as defined in the claims. While for thesake of clarity of description, several specific embodiments of theinvention have been described, the scope of the invention is intended tobe measured by the claims as set forth below.

What is claimed is:
 1. A method of determining the identification of atleast one radio frequency identification device of a plurality locatedwithin communication range of an antenna, the method comprising:determining a frequency for interrogating, wherein determiningcomprises: transmitting via the antenna, wherein transmitting isperformed in a relatively narrow band manner with respect to receiving;operating for a duration a squelch circuit that is coupled to theantenna; receiving via the antenna after lapse of the duration toprovide a received signal, wherein receiving is performed in arelatively wide band manner with respect to transmitting; determining aproperty of the received signal; and determining the frequency forinterrogation in accordance with the property, wherein determining thefrequency comprises: comparing the property to a limit; and if thecomparison is unfavorable, ignoring receipt of the received signal;interrogating to attempt to determine an identity of a device of theplurality; and repeating the determining and interrogating until eachdevice of the plurality has been given at least one opportunity to beidentified.
 2. The method of claim 1 wherein transmitting is begunbefore lapse of an expected duration of receiving the received signal.3. The method of claim 1 wherein transmitting comprises selecting afrequency band for transmission from a range of frequencies comprising afirst subrange and a second subrange so that successive selections areaccomplished to maintain a difference below a predetermined amount, thedifference being between an average transmitted power in the firstsubrange and an average transmitted power in the second subrange.
 4. Themethod of claim 3 wherein selecting is accomplished according to amethod comprising: a. for an initial selection: dividing the range offrequencies into a series of bands, the series having a number ofmembers, each band identified by an integer band number; and selecting astarting band number and selecting the substitute frequency as afrequency of the band corresponding to the starting band number; and b.otherwise: determining a current band number by adding a first constantto the band number associated with the immediately preceding selection,and selecting the substitute frequency as a frequency of the bandcorresponding to the current band number; and if the current band numberplus the first constant exceeds the number of members of the series,then revising the starting band number by adding a second constant tothe starting band number; revising the current band number to thestarting band number as revised; and selecting a respective frequency ofthe band corresponding to the current band number.
 5. The method ofclaim 1 wherein: a. transmitting is performed in a relatively narrowband manner with respect to receiving; and b. receiving is performed ina relatively wide band manner with respect to transmitting.
 6. Themethod of claim 5 wherein: a. the antenna comprises a circuit thatprovides a first configuration wherein the antenna is provided with afirst Q and a second configuration wherein the antenna is provided witha second Q; b. transmitting is performed at least in part with the firstconfiguration; and c. receiving is performed at least in part with thesecond configuration.
 7. The method of claim 1 wherein: a. transmittingis repeated with at least one of the first signal and a respectivesubstitute signal for the first signal, the respective substitute signalcomprising energy in a respective selected frequency band selected froma sequence of frequency bands; b. receiving is repeated for eachtransmission to provide a respective received signal; c. propertydetermination is repeated for each reception to provide a respectiveproperty for each respective received signal; and d. frequencydetermination is performed in accordance with all respective properties.8. The method of claim 7 wherein each respective property is determinedwith respect to each respective response in accordance with at least oneof an amplitude, a phase, and a result of a fast fourier transform of aplurality of amplitudes.
 9. The method of claim 7 wherein eachrespective property is determined in accordance with a comparison of avalue of the respective response to a normalized value, the normalizedvalue being based on a plurality of respective responses.
 10. The methodof claim 9 wherein the plurality of respective responses used as a basisfor normalization includes respective responses corresponding to atleast one of transmissions at different times, transmissions atdifferent selected frequency bands, transmissions from differentantennas, reception from different antennas, reception using differentdetector bandwidths, and reception using synchronous detectors atdifferent phases.
 11. A method of determining the identification of atleast one radio frequency identification device of a plurality locatedwithin communication range of an antenna, the method comprising:determining a frequency for interrogating, wherein determiningcomprises; transmitting via the antenna; operating for a duration asquelch circuit that is coupled to the antenna; receiving via theantenna after lapse of the duration to provide a received signal;determining a property of the received signal; and determining thefrequency for interrogation in accordance with the property;interrogating to attempt to determine an identity of a device of theplurality; and repeating the determining and interrogating until eachdevice of the plurality has been given at least one opportunity to beidentified, wherein: transmitting is repeated with at least one of thefirst signal and a respective substitute signal for the first signal,the respective substitute signal comprising energy in a respectiveselected frequency band selected from a sequence of frequency bands;receiving is repeated for each transmission to provide a respectivereceived signal; property determination is repeated for each receptionto provide a respective property for each respective received signal;and frequency determination is performed in accordance with allrespective properties.
 12. The method of claim 11 wherein eachrespective property is determined with respect to each respectiveresponse in accordance with at least one of an amplitude, a phase, and aresult of a fast fourier transform of a plurality of amplitudes.
 13. Themethod of claim 11 wherein each respective property is determined inaccordance with a comparison of a value of the respective response to anormalized value, the normalized value being based on a plurality ofrespective responses.
 14. The method of claim 13 wherein the pluralityof respective responses used as a basis for normalization includesrespective responses corresponding to at least one of transmissions atdifferent times, transmissions at different selected frequency bands,transmissions from different antennas, reception from differentantennas, reception using different detector bandwidths, and receptionusing synchronous detectors at different phases.
 15. A method ofdetermining the identification of at least one radio frequencyidentification device of a plurality located within communication rangeof an antenna, the method comprising: determining a frequency forinterrogating, wherein determining comprises: transmitting via theantenna, wherein transmitting is begun before lapse off an expectedduration of receiving the received signal; operating for a duration asquelch circuit that is coupled to the antenna; receiving via theantenna after lapse of the duration to provide a received signal;determining a property of the received signal; and determining thefrequency for interrogation in accordance with the property;interrogating to attempt to determine an identity of a device of theplurality; and repeating the determining and interrogating until eachdevice of the plurality has been given at least one opportunity to beidentified.
 16. A method of determining the identification of at leastone radio frequency identification device of a plurality located withincommunication range of an antenna, the method comprising: determining afrequency for interrogating, wherein determining comprises; transmittingvia the antenna, wherein transmitting comprises selecting a frequencyband for transmission from a range of frequencies comprising a firstsubrange and a second subrange so that successive selections areaccomplished to maintain a difference below a predetermined amount, thedifference being between an average transmitted power in the firstsubrange and an average transmitted power in the second subrange;operating for a duration a squelch circuit that is coupled to theantenna; receiving via the antenna after lapse of the duration toprovide a received signal; determining a property of the receivedsignal; and determining the frequency for interrogation in accordancewith the property; interrogating to attempt to determine an identity ofa device of the plurality; and repeating the determining andinterrogating until each device of the plurality has been given at leastone opportunity to be identified.
 17. The method of claim 16 whereinselecting is accomplished according to a method comprising: a. for aninitial selection: dividing the range of frequencies into a series ofbands, the series having a number of members, each band identified by aninteger band number; and selecting a starting band number and selectingthe substitute frequency as a frequency of the band corresponding to thestarting band number; and b. otherwise: determining a current bandnumber by adding a first constant to the band number associated with theimmediately preceding selection, and selecting the substitute frequencyas a frequency of the band corresponding to the current band number; andif the current band number plus the first constant exceeds the number ofmembers of the series, then revising the starting band number by addinga second constant to the starting band number; revising the current bandnumber to the starting band number as revised; and selecting arespective frequency of the band corresponding to the current bandnumber.
 18. A method of determining the identification of at least oneradio frequency identification device of a plurality located withincommunication range of an antenna, the method comprising: determining afrequency for interrogating, wherein determining comprises: transmittingvia the antenna, wherein transmitting is performed in a relativelynarrow band manner with respect to receiving; operating for a duration asquelch circuit that is coupled to the antenna; receiving via theantenna after lapse of the duration to provide a received signal,wherein receiving is performed in a relatively wide band manner withrespect to transmitting; determining a property of the received signal;and determining the frequency for interrogation in accordance with theproperty; interrogating to attempt to determine an identity of a deviceof the plurality; and repeating the determining and interrogating untileach device of the plurality has been given at least one opportunity tobe identified.
 19. The method of claim 18 wherein: a. the antennacomprises a circuit that provides a first configuration wherein theantenna is provided with a first Q and a second configuration whereinthe antenna is provided with a second Q; b. transmitting is performed atleast in part with the first configuration; and c. receiving isperformed at least in part with the second configuration.